i agree on most part , but themepark is the foundation of any big mmo
that is where u "hook" the player, and that is what wow did right, they had the biggest and fluffiest park
so when players got tired of endgame, they could roll an alt and really play a whole new world
any MMO dev have failed to realize that since then,,we all know the results
Themepark isn't the reason for modern MMO successes, its accessibility.
Older sandbox games were simply too complicated for the average gamer. Hell, my first month in SWG was a nightmare! Dumped on a planet with no weapons, no tutorial and no clue! Its was dire! Luckily, I had a friend to teach me and so I stuck with it, learned the game and it has been my favourite mmo to date.
Simply being a themepark is not enough to hook players. The best MMO (yet to be built) will have a combination of sandbox and themepark elements. Themepark elements are the best way to guide people through a game, for example classes, levels and linear questing. Sandbox elements are the best way to keep people playing long term, for example player cities, player made pvp wars, strong crafter-based economy, dynamic quests, random high level gear drops, dynamic / random open world raids etc.
A good example of this in the single-player world is the Elder Scrolls Series. These games include the best bits of themepark design (linear quest lines and dungeons) with the best bits of sandbox design (open world exploration, complete character freedom, strong crafting, choice of quests). As a consequence, the Eldar Scrolls are some of the most popular RPG games ever made.