The Animorphs fanbase was freakin' HUGE while the series ran. There were hundreds of websites, with a new one popping up practically everyday. I mean, it beat out Goosebumps in sales, and reached up to #15 in the best sellers list at one point. Then, of course, Harry Potter came along and paved everything along its path. In fact, I think many of the fans did switch over to Harry Potter near the end of the series, or after it ended.
I'm not sure about the actual fanbase now. This is the only site I've seen so far (after 7 years) with so many fans in one place. A lot of the other sites have really just died out over time, and people have just stopped coming. But if a movie was made, I'm pretty sure that people would still remember it and would want to see it. It wouldn't surprise me at all if it revived the series popularity once again.