Goosebumps nearly destroyed the English language. R.L. Stein can't write. His grammar is atrocious. The reason why they sold more was because it wasn't a continuing series. Yes, there were sequels to some, but not many. You can pick up a Goosebumps book at any point and get into the story. They were sorta scary for kids, which is what kids like. Also they had interesting covers that enticed kids to read. Harry Potter I like. Yes J.K. Rowling isn't a great writer but she can tell a really good story. Do people overreact about the books, yes, but I still love all the characters, and I grew up with them. People just like to hate on Harry Potter because its so popular and people get angry because they think people are jumping on the bandwagon. Now, think about this. Animorphs may not have been as popular, but who can you find who has never heard, seen, or read an Animorphs book? Goosebumps may have had interesting covers, but Animorphs always took the cake. The problem was, that you couldn't always just pick it up Animorphs anywhere. There was a whole on-going story. I think it really became about the fans in the end. K.A. may have originally written a story that she liked, but in the end she was writing it knowing who her audience was. You could say the fans had an influence on where it went, which only makes the connection between fans and series more special. On the flip side, that leaves out people who may have just wanted to pick it up. Its kinda what happened to star trek. Originally, everyone watched the original Star Trek, but later only die hard fans watched Next Generation. The connection between Animorphs and Star Trek is pretty similar too. There are a lot of references to it.
Of course that means Rachel (or Ax) is going to come back in the next book. The book following that one will of course be a Cassie book in which the Animorphs are back together and have to travel back in time, like they tend to do in the majority of Mega Morphs, to Save the Whales. Several years later we get Animorphs, The Next Generation where Yeerks now have an Alliance with humans and much better make up. Then after the series is good and dead for sure, J.J. Abrams comes along, after of course never reading animorphs, and makes a series not for the fans but for everybody else to enjoy. The End