These are businesses, and the bottom line is still profit. No profit = no game. A few MMOs have managed to stay in the pay model, which seems to imply that they've got enough of a player base and enough player loyalty that they aren't losing money, thus, no need to try a new tactic to draw in new players. The majority, though, seem to fall into this category, which is fairly inevitable: how long can you hold player interest when there are new games coming out every week and new MMOs every week, and they are coming in with the foreknowledge of what people dislike about yours? No shame, really, it's more a case of how long you can keep your head up.
I played City of Heroes for 4 years before I finally got bored of the content and decided that I did not like the direction the game was going. It held my interest that long, though the other thing that held me in was my social circle, "Team Awesome". Without them, i might have burned out earlier. Sure, the variety of characters was a major draw, but having my friends made a difference. It's no wonder, then, that MMOs are changing faster these days; when the games come fast and thick, people don't stick around as long to make those social connections as often or as deeply. Hell, I've been with World of Warcraft off and on for almost 2 years, now, and while I still enjoy it, Half of my enjoyment is playing with numerauko. If we found something that piqued our interest together, we'd probably change.
With DCU being free, I might download it and give it a look, but it'd have to be amazing to pull me away from WoW. CoH is about to go free, and friends have tried to entice me back, but remembering why I left and seeing the restrictions on the free accounts makes me less likely to bother. Hard to go back to something and find yourself locked out of half of what you enjoyed.
Just a few thoughts brought about by news.