it doesnt sound good for a community to stagnate, there is also the difference between issues in the community as a whole vs a individual having issues with changes, it would be really weird for just every single fighting game to be having problems at once
People have been playing stuff like 3rd Strike and Melee for like 25 years, community develops new strategies and tech, top players come and go, new competitive storylines form. The game stays the same but the way people play it is constantly adapting and changing. The community itself does a good enough job at preventing stagnation. Now new fighting games shouldn't all be made the same as the old, but they should at least being aiming to appeal to their existing audience. Strive got lucky and found a new audience that was willing to support it.
Every fighting game isn't having problems all that once, it's greatly over exaggerated. the only game that's truly in trouble right now is Tekken, but I have a feeling they'll recover eventually.