As long as new mmos don't try something new, it will stay flawed, and in the end, vanish again.
First of all, you have to realize that most players are bad players. Reasons are not just stupidity, but more lack of interest and the fact that they probably never touched a challenging game before.
But when designing an mmo you have to aim for a huge player base. It's the only way to keep an mmo truly healthy.
One reason: The more players the game already has, the higher chances are of new people joining.
To ensure a stable player base, you have to take care of mainly two things:
First, there must always be something for veterans to keep playing (which is a feeling of neverending progress).
Second, you have to give new players a fair chance to catch up.
Somehow, these to things aren't without contradiction. You can't do this by not taking some value out of the veteran's players progress.
This problem can be kept low by judging wisely in regards of reaching the level cap, we've already seen that. In endgame this is a whole different problem, but I'll come to that.
The resource for achieving these things is, simply put, more content.
Since this resource is limited (you just can't make the team bigger and make more content of the same quality), you have to decide for whom you design it and how much of it.
The answer seems simple: For the biggest part of the player base and we all know who that is.
Interestingly, without some elite players even the casuals would feel bored. Remember how everybody whined about the easy hardmodes without having actually killed one?
There would be nothing to achieve regarding some social standing inside the game. And if you are a game designer by heart, you want to love your own creation. So, in the end, you'll always create content for the better players, a minority.
However, this takes huge amounts of the content you can provide and there are dangerously few things you can hand out the main player base then. This is a problem.
Blizzard tries to solve it by 'making content more acessesible'. We've seen how rarely this concept works.
Patch 3.2 was a failure in regards of 'casuals'. The instance is too big for them, they are stuck at the keepers for weeks. Nerfs won't solve this problem easily. See how the new instance is much smaller.
So, in the end, you end up with the problem that a huge player base always stands in some contradiction to a challening endgame without frustration.
I know that hadn't much to do with Aion or Fallout till now, but as long as they don't come up with a new approach to this problem they'll be a wow clone at best.
And I, personally, won't play another wow. I'm rather finished with mmos as a whole, then