With all due respect, the "it's an MMO bro" and the "muh social anxiety" people are both retarded. Most people don't play WoW specifically because it's a multiplayer game, they play it because it's WoW. Some of them play it for transmog, some of them play it exclusively for PvP and don't give a shit about anything else, some of them play it for RP events, some of them don't care about any of that and just want to solo old raids for pets, some of them play it as an RPG with a combat system and cool animations while trying their best to ignore the four randoms they're forced to share their game with. You COULD make WoW itself as a singleplayer game, with the same exact progression, world, questing, movement system, spells, lore and so forth, and there are people who legitimately want that. If your argument is "go play a singleplayer game", you cannot reason why Blizzard shouldn't release THIS game as a singleplayer one (besides time and finances), especially if you just told somebody that they SHOULDN'T play it at all. You're basically looking at somebody playing the game with an offline server emulator, and saying that the game should be taken away from them and they should be forced to play the online version, because you know what's good for them.
Second, WoW isn't about loot. It's about gameplay. Loot simply exists as a means to an end, to incentivize gameplay, and provide a REASON for it. You could REMOVE LOOT ALTOGETHER, and incentivize players with cosmetic rewards instead (like many systems do already), or by unlocking more of it, or by literally making it so unbelievably fun and engaging that players will put hundreds of hours into the game just to experience it for its own sake (like MOBAs, roguelikes, or Minecraft do). The only concern with removing rewards or making them too easy to obtain is that people will become bored without a carrot on a stick to constantly chase, thus you can justify having them as an incentive to prop up the rest of the game.
Third, WoW lacks properly tuned, difficult, singleplayer experiences for the most part. Blizzard occasionally throws in relevant gameplay that is tuned for a single person (Warlock green fire questline, mage tower in Legion), but it is the exception rather than the norm. Visions and Torghast are a step in this direction, because the idea is that they're challenging content that you can choose to do with zero, one, or four other people. I would argue that the incentive to play with others shouldn't be "you're forced to play with them", with the promise of "trust me you'll thank us later", it should be to let people choose however the fuck they want to play and trust that they're intelligent enough to realize whether playing with others would be more fun for them. Diablo 3, for all its faults, is the perfect example for this. You can choose to play with others at any time, and there's a global chat system that still makes it feel like a community based multiplayer game (something WoW sorely lacks outside private servers, for reasons I still can't comprehend). You could change WoW's progression system so that it's equally as challenging to play singleplayer, and make multiplayer an entirely optional experience. Whether that would be a good or a bad thing for the game's health is impossible to say, because you'd need to actually do that in the real world, execute it properly, then observe the results 3-4 years down the line.
Fourth, a lot of people (myself included) look at the existing "tourist" modes like the dungeon finder and LFR as a way to learn the instance, and something you should be expected to do on your own time so that you don't ruin other people's fun once you're doing it "for real" (something the "proving grounds" feature was intended to do in Mists and WoD). A better and more effective version of watching a Fatboss video before the pull, in a sense. LFR and leveling dungeons COMPLETELY fail at this. People are incentivied to speedrun them because of their lack of difficulty - to the point of completely ignoring mechanics, mass pulling all the trash, treating every boss like Patchwerk, and kicking players who slow the group down or don't know what to. This is the exact argument people who want LFR removed make. In my ideal world, you could queue up for a raid with bots that behave (convincingly) like players, and practice tanking or healing an encounter on your own time for 7 hours while spending half of it alt+tabbed, possibly without even any loot. This could change things in a way that you'd be expected to do all the learning on your own, which is still a tradeoff, but I don't think it would be a worse state of affairs than normal dungeons and LFR currently are. Enabling solo gameplay as an equal option (like M+ did for raiding) would change the meta of the game to a significant extent, but removing LFR and adding a properly tuned bot mode in its place that actually teaches you the mechanics of an encounter and lets you experiment, in my opinion wouldn't. I'd even combine it with the old proving grounds system that gives you a badge next to your name, and make it AS challenging as completing a current-tier, normal difficulty raid where your personal mistakes will wipe the group and you yourself dying will disqualify you from completing it. Same with letting you queue normal dungeons with bots. If it's not more time effective than doing it with 4 other players and it's tuned in such a way that being AFK disqualifies you, then problem solved.