Deep Purple, Yes, Wishbone Ash.......... How many original band members entitle you to keep the group name.
I don't think you can specify a number to fit all bands. Some of them, after all, have chopped and changed their line-up regularly over the years. According to Wiki, for example, Deep Purple have had fourteen members, past and present, and Wishbone Ash 19. I think it's just down to a gut feeling in each individual case.
In the case of The Who, Moon and Entwistle were such unique geniuses that it was never going to be the same without them. Even The Who with Kenny or Zak on drums wasn't quite right; missing Entwistle too and it's just not the real deal. Daltrey and Townshend (performing under the banner of 'Daltrey and Townshend') can still put on a terrific show, I've no doubt. But not as The Who. Not for me, that is.