If those good deals on Steam include discounts on subscriptions it may have a chance of picking up players but, I'm on Steam all the time, 30 or so on my friends list, in a chat group with another 20 or so people and I just don't see any playing even the F2P MMOs on Steam. I've seen a couple playing Final Fantasy XIV and a few talking about WoW but that's about it. Now those aren't on Steam but they're the same price monthly and have a lot higher population, much better graphics, more updates... Having UO on Steam isn't gonna hurt but I just don't see it helping either, looking at the page for it on Steam Greenlight.