The scaling problems have little to do with rails. They admitted their problems were at the database level eons ago. I imagine "the switch" has more to do with the new chief architect switching to his favorite horse. Two whipping boys for the price of one: the old platform, and the old architect.

The new rewrite will give the new architect time to vest, until it fails, like big rewrites usually do. If you're not sure how to fix things, the big rewrite is a perfect solution, really.