OP:
No...?
Okay, this is the Pantheon forum, and there's a strong confirmation bias here, so what I'm about to say a lot of you won't agree with. But it happens to be the truth:
Themepark MMOs are emensely popular. EMENSELY. WoW doesn't track sub numbers anymore, but they were still over, what, 8 million people when they made their last sub report? And from everyone I know that keeps up with that game, this current expansion has been very popular (though considering the last one was very UNpopular might be a relativity thing going here). Final Fantasy 14 is also emensely popular. While they track numbers of accounts ever made rather than active subs, the number has steadily grown and is over 6 million the last I heard, and that game seems to be doing well, too.
On the converse side of things, there ARE non-themepark MMOs. EVE Online is probably the most famous of them, and it also has basically cornered the genre of space ship MMO (since other sci-fi MMOs like Wildstar or Star Wars: The Old Republic don't really give you much space ship combat and empire building), and yet it probably doesn't have more than 1.5-2 million players. I tend to only see around 30k online at a time, and that's the entire US game (granted, there's also a Chinese server, but you get my point). ASIDE: This number will probably go up due to the free account system they have now, but you get what I'm saying here: It's a sandbox MMO that has an entire corner of the market cornered and yet has a population significantly smaller than themepark MMOs do.
...part of this could be because not as many people like sci-fi spreadsheet spaceships as like fantasy, or perhaps EVE's legendary steep learning curve.
But there are a lot of less themepark MMOs. None of them are particularly famous or have large player bases.
HOWEVER: As with everything, there is nuance and caveat.
WoW's own devs (or the company chief, I forget) have noted that more people have played AND QUIT WoW than play it today. The implication is that, while themeparks are a strong draw to new players, there isn't necessarily a longevity to it. Of course, WoW is also over a decade old, and that could be part of it, too - some people wanted it to never change from Vanilla, others get bored with the current patch months before a new raid or expansion, and still others don't like how the game seems so raid focused where it once did not.
Themepark MMOs are popular...with caveats. They have to have a predisposed playerbase to get them going or keep them going: FF14 and SW:TOR both had brand/intellectual property appeal. TOR just had no end game and FF14...they actually did right by their players, the initial launch crashed and burned so hard that Square AS A COMPANY issued an appology to players and remade the game from the ground up. FF14 is also semi-legendary for having a REALLY player-friendly community. Some credit this to the fact it's a cutesy game and doesn't appeal to more antagonistic PvP typs or to hardcore players (though it has a small and dedicated community of each), but that probably helps a lot too, and is something that is more of a selection bias among players and not something that developers can really...make happen.
WoW, on the other hand, is king of the hill due to what I would call inertia - in physics, inertia is that "object in motion tends to stay in motion", thing. WoW got big at just the right time, and got SO big everyone knew someone that played it, so if they wanted to play an MMO, they joined the one their friend (or friends) was (or were) playing.
Most WoW clones haven't been able to repeat WoW's success because they didn't happen at the right time and because they don't have that inertia. Star Wars COULD have due to its intellectual property and fanbase, but they failed to capitalize on it well and were too much of a WoW clone. That and, as I pointed out on their forums at the time, the devs seemed more interested in making the game the devs wanted to make than making an MMO that their own players were asking for (basically an updated Star Wars: Galaxies).
Likewise, LOTR/Tolkien fans didn't want a WoW clone. To be fair, though, LOTRO does decently for what it is, and the game did have a shakeup in development. Twice. I think I read the first article on an online Lord of the Rings game in 1996 or something in a PC Gamer magazine, lol
Likewise, you have Star Trek Online, which has a decent playerbase for what they're going for, but doesn't have a lot of draw to non-Trekers.
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So the current state of the MMO industry is:
WoW the giant, EVE the esoteric, and a handfull of roughly themepark MMOs with fanbase appeal (LOTRO, SW:TOR, STO, FF14) with FF14 being the largest of those four as well as the only one of them with a monthly sub. A semi-exception is GW2, which is also free to play.
Then there are a bunch of other names I hear people say, but are so unfamous that I don't know anything about them. Black Desert which I've only heard about as a PvP game, or any number of random free to play, small time MMOs. You can't use a bunch of MMOs with account numbers measured in the tens of thousands as examples that the themepark model is failing or unwanted. The themepark MMO model is very very popular - indeed, in an era of busy people with the attention span of goldfish, it's not hard to see why it would be.
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This isn't me saying the themepark is SUPERRIOR:
EVE Online is a very healthy game with a steady population numbering hundreds of thousands to possibly a million and makes a profit for its developers. It is the closest you can get to a true sandbox MMO that I've found.
Themepark, as I mentioned with WoW, also has longevity issues. Indeed, during content droughts in WoW is when they have the largest sub losses, and Final Fantasy 14 has significantly kept growing because they have a pretty constant stream of new content (they even released a new CLASS in a content patch, not an expansion). This constant treatmill has to have catch up mechanisms, has to strike a balance between casuals and hardcores, and on and on and on. Some do it, others don't, and in either case there's a lot of player turnover.
But to suggest that the themepark isn't succesful just opposes factual reality.
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Sandboxes can be awesome too, and probably have longer LONG-term health (in EVE, to fly a large capital ship requires years of real world time for your character to train all the skills required), but will tend to have smaller overall numbers unless one just does it in a truly novel way someday and lucks into history. It's like WoW or like the United States after World War II - it was in the unique position in all of Human history to be the single industrialized nation that wasn't bombed to oblivion after two major world wars. That's unique. Likewise, WoW came at just the right time with just the right mix of accessibility, difficulty, and intellectual property (from the WarCraft games and the WarCraft 3 scenario that was a proto-MMO), as well as the right time as the MUD was dying off and people were moving to the "modern era" of graphical MMOs, and had the right community at the time that brought in a lot of players (before it became nuclear waste level toxic towards...the later end of Wrath, if I recall...)
Just recognize that, unless it does everything PERFECTLY, it's going to be a niche thing.
I expect Pantheon to have a higher potential population than EVE (because EVE is just RIDICULOUSLY hardcore and has a really nich market of those that like space sci-fi and can get over its learning curve hurdle), but don't expect more than 2-3 million players.
Which the devs here have acknowledged and are fine with.
But at the end of the day, themeparks are the path of least resistance to make money, that's why companies keep trying them.
Sandboxes can be worth far more IF someone ever gets it right. And getting it right, I think, basically would entail making a world people want to spend their time in, that has lots of options for them to do things and play the way they want. Like if you ever watched the Sword Art Online anime (the first season), something like that. Not the trapped in the game part, but the people being able to be shopkeepers or fishers or adventurers, as they wanted.
This could be done with a space game spread across planets or a fantasy game, where devs make the systems so open people can do basically anything.
...but I don't see that happening for another few decades.