So interestingly this has been asked before many years ago and the many replied there came to the same conclusion that then warcraft would not be the game for them as it is an MMO.
However the request for more solo content has always been asked on the forums, and Blizz did deliver and everything you want is literally in retail.
So lets look at your points:
Greater Accessibility: Players could experience content without Relying on Groups or Raid Schedules
This is literally raid finder in retail, pop on do a any wing you want and all content can be completed in this easy and quick mode no interactions needed
Flexible playtime: Solo Players could make progress even with short or irregular sessions
Literally everything in retail is solo-able, plus now in warwithin you have have NPC dungeon group you can complete solo.
Lower Entry Barrier: New players would face fewer social or Mechanical obstacles
Again this is all in retail you have delves that have low lvl and you decide your difficulty and they are solo-able, as mentioned above you have dungeon you can do with NCP group and raid finder doesn’t require any requirement other than reaching the ilvl which is very easy to get with all the catch up mechanics they have. All these things have low barriers and fewer mechanical obstacles and other than LFR its solo.
Learning Environment: Bots could help Practice mechanics and understand mistakes before interacting with real players
This doesn’t make sense to me, if you are solo playing why does it matter if you know mechanics to then play with real players? Surely if you’re solo then you’re solo. But as stated above NPC’s in War within in dungeons, not to mention the thousands of guides on youtube and other wow sites that tell you exactly what to do and not to do.
More stable progression: Solo Content would always be available regardless of the server population or time of day
So in retail again everything is online and you can do anything at any time of day and as stated above its all solo-able other than LFR, but i have never had an issue at 3am and tried to get into LFR and didn’t, not that happens too often.
Improved Accessibility for all players with social, physical or psychological limitation would have more ways to enjoy the full game
Again all can be done in retail, you can literally play the entire game without social interaction and same with LFR very few people talk, and lets say worse case scenario you are worried about the chat, well you can turn that off too and have a complete silence in game.
Flexible Group composition: Players could use bots to fill or replace missing roles as needed
As mentioned above already done in retail in warwithin with NPC dungeons and LFR.
Less frustration from Toxic behavior: Players would be less dependent on random groups:
As mentioned above already done in retail in warwithin with NPC dungeons and LFR and turning chat off
Stronger narrative immersion: Solo play would put more focus on story and world building
Not really sure this makes sense, the story wouldn’t change between a solo version of the game and the main MMO so either way the story is the same as you cannot have one version differ from the other, so again in any version of the game this stays consistent and you do the story and side quest as you want.
Longer Lifespan for older content: bots could make old dungeons and raids accessible again even if real groups run them
Again this is all relevant in retail MOP classic prob not ERA or Anniversary as there are no old dungeons. But you can solo all old dungeons and raids, not to mention now with player housing you have to go all over the world and to old expansion to collect the housing patters and items. Again all solo-able.
Reaching new audiences: A solo version would appeal to player who avoid MMO but love the world and story of wartcraft
This doesn’t really make much sense, at the end of the day as someone else has pointed out people that would only want solo content is small, and if you are a fan of the the game, a game which is now 20 years old you already know the deal and there is lots of information about how friendly retail is to solo players i can’t imagine it being the deterrent you are making it out to be.
Stronger player retention: Players with irregular schedules could stay engaged without becoming frustrated by group requirements
Again all in retail and there this is pure speculation that you think a solo version increase player retention. There is no group requirement in retail other than LFR for content. But if you try make the point of doing M+ or mythic raiding this should not be in a solo version of the game because there is zero need to have the more difficult content and as you are trying to make out you just want to do the content at the lower difficulty and accessible.
Better use of existing content: Decades of quest dungeons and raid could remain relevant through solo adaptations
Again this is done in retail refer to my above answer about player housing hunt and players keep content alive if they want to do it because they are chasing transmogs or achievements.
Independence from server population: Content stays playable even when certain realms or time slots have low activity
Not an issue in retail as mentioned above all the majority is solo-able and can be done at any time of the day.
More accessible game play: Players with social anxiety or physical limitation would gain fill access to the experience
Again in retail nothing stopping anyone from playing the whole game without the need to interact with others.
Improved learning curve: Player could master classes, rotation and mechanics without social pressure.
So this is again in retail and exactly what the dungeon NPC runs are for, you run these in a spec you dont want to run with others, Tank or healer and learn, but furthermore you now have single button rotation where you just literally play hitting one button, no need to learn rotation just spam, this is the most accessible feature for all players with physical disabilities. There is even rotation helper in the retail version to help you learn.
Reduced Strain on the community: Fewer Frustrated players in random groups since solo players wouldn’t be forced into them
Again all in retail as stated above, but what i will also add in retail there are also fantastic communities set up specifically to help anxious players. Communities like No Pressure EU and scared of dungeons that welcome players who are nervous, anxious and worried about doing the harder content. I have run many M+ in these groups and there is never any toxicity, everyone is positive and helpful and helping those players get through the tough parts of a Mythic plus. Helping with suggestion and tips in the community chat also.
Testing Ground for new systems: Blizzard could experiment with features without disrupting the main MMO environment
This is just silly on so many levels, there would be no logical reason to test new features in a solo version of the game even if it existed. Firstly that’s what the PTR realms are for so you don’t need to test it on a live version of solo game. Secondly you need volume, and I agree with the other comment that the solo version won’t have the numbers playing you think it will. With the current PTR and Beta for expansion and patches that Blizz gives you access too, there is a huge amount of players that are playing and effectively being testers for blizz so this doesn’t make any sense at all
Expansion of the franchise: A solo edition would diversify WOW, similar to how classic and hardcore added variety
So yes they added variety but they are the same version of the game, both are MMO both have the exact same quests and raids and dungeons, the only difference is one you die in you can’t play on that serer again. You could add SOD and SOM also and MOP classic, but what tends to happen is those version become less populated and played, having more options is not always a good thing.
Long-term value: A separate version could bring players who enjoy the setting but dislike group play
You already said this in one form or another in the points you made, and i’ll say it again retail covers the majority of what you want
TLDR: Go Play retail it has everything you ask for with a caveat of LFR but you can play with no social interaction.
So you have the options to play wow as a solo player whether you choose to is up to you of course, and as has been mentioned if not then this might not be the game for you as there are lots and lots of solo RPG games out there that would offer what you want in how you want to play