All kinds of MMOs are going free to play and being relatively successful doing it. In fact it is almost a sign of a seasoned MMO these days that it finally makes the transition from an unsuccessful subscription design to a successful free to play design. SW:TOR is going free to play, obviously hoping to cash in on people desperate to hand them money for sparkles and shiny, but they managed to bork the transition pretty successfully. Turns out they got the numbers wrong... not exactly a first in MMO design.
Free to play sure isn't any kind of panacea though. Glitch has been running that since day 1 and they just announced that the game is shutting down permanently. Despite having a pretty cool subscription option (getting to vote on game changes is neat, though it probably didn't amount to much) and a world that was quite a bit of fun the game still went down the toilet. You certainly can't expect to make a bunch of money just by producing a game, letting people play for free, and hoping the dollars roll in. It makes me a little sad, even though I haven't played Glitch for a year. It was neat, and fun, for a time.
I think that the subscription only model is likely to vanish from MMO products (with the possible exception of Titan; Blizzard's brand may be strong enough to support it) in favour of free to play with subscription options and microtransactions. Letting people log in here and there to see their friends and keep their addiction alive is a good way to get them to pony up some cash to make the game smoother and better. That isn't going to fix the constant stream of mediocre MMO products that crash and burn by any means; people still have dollar signs in their eyes when they look at WOW and that leads to all kinds of duds being shovelled out there hoping for a big score.
On the other hand Free to play tabletop RPGs seem to be a thing that works. Pathfinder has all of their rules sitting on their website freely available and yet they are still raking it in from their book sales. Wizards is envious enough of the cash Pathfinder products are making that they are desperately trying to design a new edition to recapture that market segment and casually tossing the fans of 4th edition DnD under the bus to do it. It is pretty clear that offering the basics for free and offering convenience for cash is a great model but you have to have a strong product in the first place.
Same thing with music downloads. People that download lots of music illegally also tend to buy a lot of music. People that sample music from bands also buy music from those bands. If you make something that people like you maximize your profit by letting them try it and then taking their money. A few people will just scam you, but plenty of them will end up buying to keep you very well fed.