I agree that PVP gear rewards really need some rework, but I have yet to be convinced that Mythic+ ruins raiding.
Firstly, Blizzard has stated that their internal data shows that there is a huge overlap between those who raid and those who run Mythic+. In other words, players who only run Mythic+ while occasionally raiding are very niche. The idea that Mythic+ is discouraging raiding is a myth.
Secondly, to address the inevitable “but 415 gear” argument, gear is more than just an item level. Your best items are probably not going to come from Mythic+. Your best or most optimal items are probably going to come from the Dazar’alor raid, as it does not share loot tables with Mythic+. Mythic+ dungeons have loot tables that rarely change if ever in an expansion. Additionally, Mythic+ does not drop Azerite gear. Instead, you have to work for that gear over possibly multiple weeks through Titan Residuum that you get from the end-of-week chest. That gear is expensive.
Thirdly, while this is minor, Mythic+ does not drop Breath of Bowansamdi, which is a requirement for advanced crafting recipes.
I know folks want to pit the blame for raid burnout on Mythic+, and also might ignorantly see the “415” item level and think that is unfair, but the reality is that raid burnout is a symptom of a different problem altogether.