The first big change was when the music was first recorded.
But to your point,
I'm not aware of any pioneers who labeled Rakim, KRS and Kane as not hip hop. There was an issue with gangsta Rap because of content, but a contention that it wasn't hip hop? I don't know about that.
@IllmaticDelta wouldnknow the answer.
At the end of the day, we can't call everything hip hop. Is there some reason the mumble singing hasn't to be called hip hop? Most of the proponents of that music view the 90s as dusty anyway.
1) it doesn't have to be pioneers, just listeners, as you are. someone who was fukking with hip hop in 70's - mid 80's prolly looked at it in 96/97 and said wtf?...same thing you're doing now.
2) cats like bone, nelly, 50 and ja opened the genre to being inclusive of singing/melodic rapping so i'm not sure why the beef is with the kids doing it when the old heads opened the gates. if what they did got to be rap or hip hop, i don't see the difference now, they're rapping, regardless of if they mumble thru the bars or "sing" parts of them (don't take me as c/s'ing all the wack talent now...but you seem to have one view of of rap and that's pure bars/spitters/rappity rap cats, rap hasn't been
just that for ages)
Edit: last point, look at rock - there's death metal, hair metal, light rock, classic rock, alternative, acid, punk...and so on, it all gets rolled up under rock. no reason why trap or mumble rap

can't be sub genres that live under the greater umbrella of hip hop