A band is not a band...when the fans stop showing up for concerts...
and buying their albums.
The fans really decide.
There are some bands, I think who aren't the same anymore because of different members have joined the band...
then there are bands, where the band isn't the the original band, because members have passed away, quit the band, or whatever, but still sound good in concert and it pleases the fans...
For instance, I wouldn't go see Yes with out Jon Anderson or The Allman Brothers Band with out Dickey Betts...
but, I would go see Lynyrd Skynyrd, because they still sound good, make decent albums, although most of the band has passed on...Skynyrd still packs 'em and still put on a top notch rockin' show.
Also, saw Deep Purple once without Blackmore, yeah, it wasn't really Purple, but they also put on a top notch show too....matter of fact, saw them with Lynyrd Skynyrd...fun concert. (but then again, not a fan of Purple albums with out Blackmore, to be honest, but Deep Purple still puts on a great concert!!)
Again, in the end, it's really up to the fans to decide when a band is not a band. If you like it - great, if you don't - that's ok too.