To change my discussion from pay to win MMOs, we'll look at sandbox MMOs and why there have only been three successful ones and all else fails.
So, lets look at singleplayer games to start with, compared to battle royale. Granted completely different games and genres...but there is a good reason battle royales are made and not high quality games like Witcher 3.
Witcher 3 cost millions to make, its budget was extremely high. Voice acting, graphics, story, real gameplay content made by the devs, the world.
Battle royale games you design a tiny map, no voice acting or very little, gameplay content is made by the players and very little by the devs.
That last part is why battle royale games are made. Cheap to make because they don't need to provide content if the players make it for free.
And that is where the discussion of why so many sandbox MMOs fail. That and all are forced PvP with no safe zones, which is one massive reason Ultima Online, Black Desert (well exception to this cause if you level too far its pvp forced) and EVE Online (the three successful ones that still officially exist without going to private/free servers) are the only sandbox MMOs that have succeeded in the genre. While EVE you CAN be PvPed anywhere, generally they made high sec safer and low sec very dangerous, then nullsec is actually safer in my experience for some reason. And BDO the pvp is so casual with no penalty to dying in pvp at all, that it is worth listing.
So that is one reason. The other...content. And this one is more important than safe zones or casual pvp content or whatever.
Ultima Online, BDO and EVE Online all provide an immense amount of content. UO has an amazing housing system, crafting, dungeons, tons of PVE content in general. EVE Online also has a ton of PVE content and many players don't even PvP despite it being considered a pvp MMO. BDO is the same way, the vast majority in BDO don't pvp either despite it being forced pvp at a certain level.
The three sandbox MMOs spent a lot of money (and time) to provide quality content to the players. Look at all the failed pvp sandbox MMOs...the only content is "players make the content". Code word for "we are an indie company that can't afford quality content, so we are gonna make a B-rate MMO and make the players provide the content for us". And this is where they fail and the companies and devs that can actually make a good QUALITY MMO win out...
Sadly, indie companies love their sandbox MMOs despite so many failing, because they think players making the content is free labor and they don't need to spend anything to make any other quality content. That makes them B-rate MMOs and deserving to fail.
Weirdly, they ignore the success of UO and EVE and keep making low budget MMOs with little quality content made by the devs.