If that's the number they're aiming at, I'd say they have a good chance of success.
Why?
Theme and market. Bioware games sell millions of copies routinely. Star wars has one of the largest audiences on Earth. Their intersection is probably at least a couple million, and if the game is good enough to keep their interests, it should have no problem maintaining half a million subscribers.
If it's comparable in its base elements (obviously not content, at least not this early) to World of Warcraft, they might have a chance of tearing a nice chunk out of WoW's playerbase - people who like the MMO gameplay, but would prefer a scifi/star wars theme. This would be a real challenge to maintain, however, if the game doesn't release in absolute peak condition.
And considering that EvE Online, an arguably nigh-unapproachable niche game, maintains upwards of 300,000 players, if this game delivers on its promises it should have no problems whatsoever in holding the line.