Sure, making a group for M+ is slightly annoying. But most players who do them from my experience is people who wants to either do a weekly +10 for loot or those who enjoys doing content with friends.
Neither of these hugely benefits from solo queues. Not to mention the very basis of the M+ system is the keystone. You would have to scrap that system, meaning the weekly run would either be based on RNG, in which case you would want to make as good a group as possible. Or you could choose which dungeon to do, in which case everyone would choose the same 2 or 3.
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50% is a perfectly healthy amount of players raiding.
You would have maybe less than 10% of players who have yet to get into any form of raiding. Maybe 10% taking a break and maybe 30% who exclusively do PvP.
Raiding is, i would imagine, still the biggest single block of the playerbase. Raiding is also the best draw Blizz has in the MMO market, no other MMOs manage raids on the same scale as WoW, not to mention that much of the lore of the games are centered around raids. Questlines lead up to them and dungeons are buildup.
Not to mention Raids are the most important endgame we have. Either that or high-end PvP. Gear is useful almost solely because raids require ever higher levels of gear.