There is no real top-notch Sci-Fi based MMO's out there. What is out there is weak. A Starcraft MMO would clean up, especially in Korea and the Asian market.

If Blizzard gives the option to bundle the monthly fee for both WoW and the Starcraft MMO, they can really make a nice profit without damaging the WoW player base too much. But if they make people choose, WoW will take a large hit in player base.

But the rumors are not so far fetched, especially if you consider how Warcraft III set up WoW, it's likely that Starcraft II will set up the Starcraft MMO.

Personally, I would find it hard not to move to the Starcraft MMO, it's far more interesting. And if it's has more integrated PvP like Warhammer Online, rather than being an after thought and side game for free epics like in WoW, then it would really attract a lot of players.