I don't think that the success will hold on. WoW is loosing subscriptions for years now, lotro was forced into F2P and StarWars, well, it is more or less a flop and this with such a setting from bioware ! What about all the other games, DnDO, Rift, Aoen, Secret World, AoC, all have been beaten by WoW a 7 year old game. I think that WoW is still attracting new, more casual gamers, whereas core gamers keep away from new MMORPGs.
All in all, my impression is that the vast majority of people doesn't really have too much interest in rich RPG and in fact enjoys a theme-park style of game, and even "grinding" (even while complaining, they enjoy it). I couldn't explain otherwise why such games are so massively successful.
With elder scroll online we will see if an other top single player developer is able to pull it off.
Skyrim (for all it's faults) could teach these MMOs a thing or two. Open the whole world up and let players have missions that span the world. Make them take time to do.
The risk of developing a MMORPG is very high and many developers have tried to get their own MMORPG on its way, but many failed the desired goals. So, why do all try to make their own MMORPG ? Well, it is like always cash, which is greed, which is a bad base for good game design. The latter is reaching its climax in F2P MMORPGs:
grind as long as you want, but you need to pay to have fun