Without going into specific MMOs, it is difficult to talk about why certain MMORPGs crash. The biggest problem of the last decade was developers copying WoW's system down to the letter and trying to put a new coat of paint on it. SWTOR was a monumental project with a lot of hope behind it that turned into the colloquially named "Tortanic" due to every aspect of its gameplay being a direct WoW rip off. The real issue though, came after the release, where the developers blamed the quickly bleeding player base on a lack of end game content, when clearly many of the players were leaving before they even reached end game.

Because we have all played the same system for so long, people who regularly play contemporary MMORPGs know at some level the problems with these kinds of systems, probably even moreso than the people designing the games. One of those is the upwards progression associated with WoW. The thing is, you can make a game's content have lasting value, since every other genre of game does that. Someone who beats Skyrim will get the same satisfaction now that people who beat it back in 2011 did. Certainly they have fixed a ton of bugs and problems since then, but the experience is just polished, not permanently altered or changed. Not to mention they also add new stuff without irreversibly changing the old.

It also hurts the RPG elements of the game to constantly expand upwards. If a sword is supposed to be a relic of great power, why would it suddenly become irrelevant statistically in the story the second Leviathan is encountered? Must we ascend an endless ladder of supposedly priceless holy relics apparently each more powerful than the last for no explicable lore reason just to down a boss? MMORPGs already have enough story telling problems as is.