It would bring a ton of people, on that I would be willing to bet any amount. It doesn't matter if it's outdated. People are bored, looking for something to do for a night, don't want to buy a 30 dollar game with monthly 14 dollar subs. Now give that game a free option, stick it on steam... many bored people will give it a whirl just to kill an evening, and many of those people will end up thinking its a cool game. That's just how f2p works, that's the entire point.It's too late for that. How many players would a shift to f2p or b2p bring?
The client is out of date and people laugh when they see the game. They wouldn't touch it with 10ft pole.
There are many games with far worse graphics and far worse gameplay than UO that have 10x the players.... why? No entry barrier, no other reason.