You could find ton of examples proving on behalf of whichever side of argument you support.
Having said that, it is undeniable every single larger sucess story on MMO market within last few years has been flirting with F2P in one way or another. Lord of The Rings Online, Dungeons & Dragons Online, The Secret World, Star Wars, The Old Republic. All of these games started sub-based. All of them eventually went FTP. All of these games have devs suggesting heavily it was F2P that saved their game. Two of the most recent examples on that list (TSW, SW:TOR) are EA games just like UO. One can very easily say EA has very positive experiences of turning sub based MMOs into F2P.
Due to it's sandbox nature, housing and, most importantly, itemization I believe Ultima Online would make an excellent F2P game. It is already halfway there, really. Many functions and mechanics that work as honeypot in F2P have already been brought to the game. (We have had a cash shop for years.. ) I find it likely it'd bring tens of thousands of brand new people to the game. ) However, since it is getting very questionable wether this game actually has a live dev team anymore, obviously it is not something we ever have to/get to worry about.
I believe lack of resources,lack of devs,lack of PR muscle are the things keeping UO from going F2P. Not lack of desire among the remaining handful of it's devs. It'd take a major development and advertising push to pull it off. Hell freezes over come day UO ever gets either of those for any reason;(
..And yeah, when it comes to numbers of WoW it is good to remember the game is 10 years old soon. Being able to hold 8 mil active subs 10 years after release is utterly crazy. To put it in some perspective, Eve Online and SW:TOR, which I guess are both in top-3 most popular western MMOS, have around 6% - 8% of WoW's subscribers.
....Typing that, I realized WoW is as old today as UO was in 2007;o Yet small part of me still views WoW as 'that new thing' When did I get so old