I am not sure that I agree with this argument.
I have never been interested in WoW and I never will be. However, I really enjoyed SC and I plan on buying a copy of SCII, as well. So why would Blizzard want to refuse my $60 when I am not one of the people who pays them $12/month?
On the other hand, if your argument is that WoW players will, all of a sudden, stop playing WoW and stop paying their monthly fees so they can play SCII, I don't think it will happen. Not if we are talking about players that have played WoW for long time and invested all that money and time into the game.

In my opinion, saying that Blizzard is not interested in releasing SCII so they can keep milking WoW is the same as saying that Blizzard is not interested in more of your money for SCII until they can't make any more money from WoW, which is just not true.