Some random history here, but this isn't the first time blizzard have split a game up in to many. (According to my conspiracy theory)
Random Fact: Warcraft 3 was meant to have
6 factions, that was cut down to 4. If you look at the ROC and TFT you can see that story elements from the game have been moved between factions.
It was confirmed that the Demons were meant to be a playable race. And the other is unknown. Looking at the story campaigns throughout, it goes like this:
ROC:
Human
Undead
Orc
NightElves
TFT:
Night elves
Human (blood elves)
Undead
Now if you remember the 2 races that were dropped, namely the demons and "some other race", you can actually fit them into the story as one game and one story pretty well:
Human
Demons (instead of the undead, demons ravage the eastern kingdoms)
Orc
NightElves (merged into 1 campaign)
<unnammed race, Blood elves, or the Draenei - but I say the Naga>
Undead - where Arthus wins
Of course, because they dropped the 2 races, they just recycled some old ones to replace them in the story. Although I don't think it was profit making, as I tend to agree that the demons should be powerful (and hence hard to balance in multiplayer) I think the reasons for this split and change was to set the scene for WoW. But it worked well, so perhaps the idea to split SC2 into 3 has anterior motives than to make money (seeing as blizzard print money as it is...). Maybe it's to keep the excitement of SC2 until "world of starcraft" is ready in 2 years time?