I was just thinking about all the Early Access MMO's out there these days, and how much I hate it.
I don't know what the numbers look like for full launch vs. early access, but I definitely prefer full launch. I can't even start to get excited playing an Early Access game, and I don't think I'm alone. People inevitably review it and rate it poorly for dumb shit that comes with being 'Early Access', and it creates a death spiral.
The ones I've seen all seem sparely populated. Development plods along even slower than it did prior to launch, because now they have to juggle a live server, and eventually the interest fades. I think your average player will try it, move on, and never come back.
At least with a full launch, you have an initial population surge and they start convincing each other to play. Creating/maintaining a healthy sized community is the most important aspect of an MMO. Especially if they are trying to go subscription model.