I agree that it was a mistake. MS should be embarrassed of the fact that they don't have a AAA PC studio.Yep a lot of things can change, like MS realizing that their strategy to sacrifice their PC studios to focus on Xbox was the wrong one.
I just wanted to point out that the timeframe between Enseble's shutdown and Decisive Games' founding is a bit longer than you said it was. They didn't have plans for an RTS then, but they do now.