I certainly would have preferred there be more guerrilla groups to add some verisimilitude to the story. What I mean is that events play out like a children's story... you know, because it is. Kids are outwitting adults who are far more powerful then them but are far too clumsy and oafish to win. As protagonists, they can be highly successful, or only a little successful, but they can't be outright defeated or else it is all over. Now there are several reasons, from an author's perspective for building the story as it is.
For one, building the fictional plot to be more realistic might also make it too complex for a young audience.
Another: if they had a lot more going for them in terms of others resisting the invasion, it would take the pressure off; the author obviously draws on the "we are alone in this resistance, if we lose this it is all over" to build suspense (This, to me, is unsatisfying. You never have to wonder until the very end if someone isn't going to make it out alive: At one point Tobias dies. He is simply dead, he is assuredly dead, the book ends and he is still dead. Yet I still don't find it a spoiler to say that no, he wasn't actually dead because even as I read it I knew he wasn't dead. Why? Because it was too early in the series and the main characters simply couldn't die until possibly near the end. Still, though I call it unsatisfying, I recognize that an author may deem it necessary to provide the suspense: It wasn't just about your fellow teammates, but every risk is gamboling with the fate of all of humanity).
So, understanding that there were reasons for the story to be as it was, it is still notable that it lacks in verisimilitude in this respect: The Yeeks would have won, and would have conquered Earth a lot sooner. It was necessary for them to start slow, in the manner that Visser One devised, building a front organization and gathering a solid group of local hosts in secret. This way, they didn't have to ambush with alien bodies every time they wanted to take a new host because once the secret got out there would be all out warfare, the Yeerks might still expect to win in the end, but at the cost of not only many of their existing hosts, but the most valuable resource of all: billions of very capable host bodies. But after that, at the point where the story begins, things should have been much quicker, not a gradual infestation like we saw through the series but an explosive, exponential increase. With a starting head start of 'hundreds or thousands' that the Yeerks had, it should have been a daily affair for a couple of Human Controllers to a mutual acquaintance alone in a secure private location, then spring a trap, secure and infest them. Unlike a ring of kidnappers, they wouldn't have to worry about striking too often or in the same location because there would be no one missing or behaving different to cause the non-infested any alarm. An incompetent, ego-maniacal leader like Visser Three running off engaging in isolated skirmishes with five to six Andalite bandits on a daily basis would be irrelevant because with an exponential spread to the infestation the Earth would be taken over regardless of the fact that a logging operation got shut down and a ground-to-space cannon got destroyed.
It's like the word problem: "If someone offered to give you $100 today, or offered to give you 1 cent today, 2 cents tomorrow, 4 cents the next day and so on, doubling the amount they gave you for 30 days, which would you choose?" The $100 sounds like a lot until you realize that the one with exponential growth becomes 10.7 million dollars. Who cares even if the Andalite bandits managed to run a mission on a daily basis and blew up important things all the time and stop important operations each time, it really wouldn't take long for 2 or 3 controllers laying a trap and infesting someone on a daily basis to take over the Earth. The only really significant event early in the series was the destruction of the Earth-based Kandrona, pausing the infestation long enough to withdraw as many as possible on a 3-day basis and tighten up security on escaping humans for a couple of weeks.
The only weak explanation offered for why the takeover moves unrealistically slow: They 'prefer voluntary hosts,' a preference that isn't that relevant when we see that an involuntary host is almost always a minor annoyance but not a real problem except in rare cases; when they need to drastically boost their host population and gather much more constructive and fighting forces for an all-out galactic war that 'preference' would soon be forgotten.
If, on the other hand, books assumed and explained early on that the growth was going to happen at that ridiculously fast rate, the way it would have realistically happened, unless something stood in the way. Then, the Animorphs could have found the cube early on (or even fled the construction site with it) and began establishing many hidden cells of fighters and spies. This group would be able to grow only at a slow linear rate, rather than exponentially, because whoever was in charge of the cube would only be able to add only so many allies a day. But if the Yeerks knew that there were not six morph-capable Andalite warriors but a gradually growing underground resistance of morph-capable and trained humans, knowing that spies were constantly patrolling public spaces, following suspicious activity and ready to call on battle-ready reinforcements spread throughout the city and always within a few minute's travel would force them to be much more careful with when and how they take control of someone.
All of the logistics of all-out warfare would be unnecessary if it was kept as guerrilla style warfare, drawing on local, non-military population and training them in small cells, prepared for hit and run combat, like traditional guerrilla warfare is done warfare is done. Applied to this case, they could be capturing individual known controllers and starving the Yeerks out of of them. This freed human could not return to normal life because Yeerks may recognize that he was a Controller; but this individual could grab any liquid assets they have and go into hiding. Free individuals could be pulled aside and taught enough to be brought into the secret war (after also being set aside for three days to be safe) to help develop safe-houses and underground hide-outs throughout the town. They would always be contacted by existing members in changing locations but never revealing the human identity of who was training them. They would be trained in the small, convenient groups of 5 or 6, only given the contact information a few other groups for emergency situations but never enough information to directly give away another group that is ready to disappear from their location in a minute's notice, and ultimately being ready to live by the 'Freedom or Death' policy to keep morph capabilities out of the hands of the enemies.
Now, I know I've over-thought this way too much for many people's preference, and it probably sounds like I'm attempting to re-write the entire series to my own imagination. But what I'm thinking of is that early on, the Animorphs could have found and entrusted an adult with creating this ever growing ring of isolated cell and fighter spies, but spread throughout the town and work as hidden watchmen over the Yeerks growth. This way, then our main characters could have proceeded as they did, but able to call on additional ground support and resources on occasion (not battle support, because they can't go yelling for help in open-thought speak to everyone within a few blocks all the time, but it'd be nice to have someone drop off a few pairs of shoes and clothes in advance). The main difference is that they would be aware that if they died it wasn't the end of the world (literally), though their missions were striking blows toward the various strategic operations of the Yeerks to slow them down, there would always be various individuals slipping in and out of societies as humans and watching over them as all sorts of subtle animals. None of this would have to be the central subject to the books, but could have been established in the background with occasional communication with that entrusted adult who is managing all the of it. It would take away a bit of the "All of humanity is at stake and about to die... no wait a gull got sucked into an engine and crashed the helicopter, we're good" suspense, but replace it with a much deeper and more solid verisimilitude.
Don't bother telling me "Dude, it's just a book," I'm aware, I just like to muse through possibilities that could make these fictional worlds stand in their own right. Also, I mean none of this as criticism to the author; hindsight is 20-20, so it is a lot easier to look back and say "it could have been established like this" to stand more strongly, when obviously there was much to the Ani-verse that was established along the way and not intended from the beginning.
