Always loved the ending, personally.
Katherine sticking by her guns and raising the middle finger at the backlash in that really eloquent letter-to-the-fans sometime afterward was really, really awesome too.
It's not a "cliffhanger", either, that term sort of implies there's more coming and you have to wait extensively for a resolution. This wasn't that, the ending
was the resolution. In terms of this particular story/phase, the Yeerk/Human (and even the Yeerk/Andalite) war being done.
Also, you don't need to have any context for The One, know anything about it or its goals. It's a narrative device, nothing more - that doesn't make it forced, or unearned. What they're getting at is, for someone like Jake, the fight is never over. Their goal in saving Earth might be done, but if a friend's in danger he's going to rally the troops for one last mission. It's just that classic western/gunslinger thing.
Which is why personally I love the interpretation that they all went out like Big Damn Heroes trying to get Ax back, it's just way more satisfying in a weird way if they all die. Cassie's sort of the only one with much left to offer at that point anyway, even Marco sort of acknowledges that he's basically lost and just filling his time with money, fast cars & women, doesn't know what to do with himself.
Think of it in terms of, like, Logan recently. Weathered disillusioned dude coming around to do the right thing, sacrifice himself in a last hurrah to save a bunch of kids. Animorphs isn't exactly the same thing, but it's a similar type of trope: the war's the biggest, most important thing they'll experience in their lives, they've "peaked" at like 17, and now two years later (three? been so long) they're all given this chance to come out of their funk and go help an old friend.
They might have succeeded, they might not have. But we
don't need to know which, you can draw your own conclusions. That's not a cliffhanger, that's just smart writing and not treating your audience with kid's gloves. Personally, my take is that Ax was dead and assimilated long before Jake & Pals even showed up, and that they all probably suffocated in the cold void of space in a last hurrah.
Which is, narrative-wise, awesome.
Of course Jake frickin' Berenson, the Boy General, is going to go out fighting. Plot-wise there's not much more he had to offer anyway, he's the tragic youth hero figure, his arc was done.
