Would more be better?

Today the head-start period for Rift begins, and I have no idea how well it will do. But I do know that Rift is not completely unlike World of Warcraft. It has the same basic "theme park" guidance by quests structure, it has classes, levels, talents, spells, and a combat that works very similar, and even the user interface is somewhat similar to that of World of Warcraft. I also know that Star Wars: The Old Republic will also fall into the same basic scheme.

That is not necessarily a bad thing. After all, everybody can with a similar degree of accuracy predict how the gameplay and controls of Crysis 2 will look, without people blasting it as a "Half-Life Clone". But it opens up an interesting question: Imagine that Rift and SWTOR and a couple of other games manage to reproduce the same quality of World of Warcraft, and all have a similar gameplay, with minor variations (Rift has rifts and the soul system, SWTOR has light sabers and space ships, another game has another setting or added feature). Would we be better off in such a world with many MMORPGs with minor variations all being similarily successful?

The reason I'm asking is that I can easily imagine me playing my WoW priest for an hour, and then switching to a SWTOR Jedi and play another hour. But then I also have two servers full of alts, Horde and Alliance, and already frequently switch character during an evening. And I know that other players prefer having one "main", with alts just being there for bank storage or crafting. I have a hard time imagining somebody who doesn't like alts being all that willing to switch frequently from one game to another. Furthermore, given the monthly subscription business model, playing several games in parallel can become costly.

So how does a future look in which several MMORPGs have multi-million subscribers? I don't think the "WoW Killer" scenario, in which one new game rises to several million players while World of Warcraft crashes to under a million subscribers is even remotely possible. If we want other games to succeed on a massive scale, we need to be able to imagine several of them living side by side. It certainly works for most other types of games, but are MMORPGs somehow special due to their larger time requirements?