every game is going to have something you don't like about it eventually
I will not hear this slander toward Darkest Dungeon!
Except for it is a game outside of PVP as well with levels and progression and gear, so it's more like just a different PVP system. Did WoW stop being an MMO when it tried (kinda poorly) to move in that direction w templates?
I think WOW has busted a lot of the traditional understanding of what an MMORPG (and especially the RPG part) is and should be. For better or worse, I wont pass judgement there. But you're hitting Ship Of Theseus questions about what is essential to an MMORPG and what can be replaced without changing its nature.
MY argument is that essential parts to MMORPGs are character leveling, representing personal progression, and acquisition of gear, representing mission/story/campaign progression. Theres obviously some overlap there and some variations but ultimately this is how MMORPGS (and RPGs before computers) progressed.
As for whether or not changing this in pvp fundamentally changes the nature and genre of the game, I think first you have to ask what role PVP plays in that game. If its a small sideshow (like it is in Diablo, for instance) then do whatever you like. But if you want it to be a major factor or draw of the game, then I say it does fundamentally alter the game. For us twinks, this is why I generally argue against things like templates and gear scaling (while acknowledging that Legion pvp wasnt too bad!). PVP is a huge part of our experience in this MMORPG and causing the gear to not matter fundamentally alters the type of game we're playing.
If I'm going to play a combat game against other people where gear doesnt matter, why am I playing WOW? Arent there games that achieve this goal in better or more interesting ways and with way less feature bloat? If its because I care about my character, do I not also care that the progress I made count for something?
I know the counterpoint is that we dont want gear to carry bad players against good ones (which I sympathize with) but contrary to the OPs experience, I'd say thats not really happening. See Exhibit A, me. My warriors are pretty fucking well geared. Hell, even my alts are better geared than like 90% of the people on the armory. And I still got my ass handed to me by guys in enchanted heirlooms in the arena league because they are all actually good at the game.