More for a general discussion than regarding the bands mentioned, or ones like them, but why does a genre need to be time-capsuled, locked, or contained? Why does something coming later have to be a rip-off?
It seems to be only certain genres, like Punk, Metal and yes, Grunge that people seem so touchy and precious about. Like they can be the only 'true' representatives of a genre or era. Seems a bit self entitled to me.
You don't see Jazz contained to the prohibition, people still do jazz today. Or Folk to the Burns years (well before, obviously for both, just picking the most obvious ones), or rap etc. But someone taking influences from one or two genres popular amongst a much smaller group, and you get dismissive purism.
More a general observation than particularly aimed at you or what you said, and unrelated to the couple bands discussed. Which i wouldn't acrually disagree could easily be classed as a 'rip-off'. Just don't see what's so wrong in that, it is pretty common.