They've been waiting for the right push for over a decade. They no longer have the magic they once had.
Most studios (and companies) have turnover. Everyone loses top talent. It's about the infrastructure and leadership to keep replacing top talent with more great talent. Rare slipped because MS was managing all of their studios and gaming division terribly for a time. I never purchased Viva Pinata or Kameo but they were well received. Rare lacked direction when MS did. Their talent that left indicated as much.
If MS wanted a studio capable of developing a AAA platformer, they have the resources to hire them. They put together 343, Turn 10, and Coalition together from scratch. Unfortunately they didn't have a focus or identity for Rare other than the Kinect studio. That chapter is over.
Now that MS has lost their obsession with Kinect, I fully expect to see a Rare comeback. The guy in charge of this next IP was a designer with some of Rare's best games. I see no reason why MS would go cheap in putting together the rest of his team. Spencer seems to understand the potential upside and value in a Rare comeback.