I think you're spot on with the mini-computer to PC transition. This carries all the same echoes of the frontier-days of early PCs. Initially, people (myself included) wrote tablets off as being a toy for enthusiasts, and now they're reaching a level of adoption that's shifted the question from "Why would I buy one?" to "What's the best one on the market?" which can only be good for consumers and computing as a whole. Microsoft ended up winning PC's for a good twenty years, and have gotten destroyed by Apple in terms of branding and attention in the past twelve years or so. We'll see how they do in the teens.
I don't think people realize how big this announcement was.
Yes, MS is still a multi-billion dollar company and a good portion of their revenue is likely secure for years if not decades. But we are at a key juncture in computing here, as important as the mainframe/mini-computer to PC transition if not even more so. And if MS had just blundered on and made half-assed token efforts they would have forever set themselves into the wrong gear and they would have relegated their company to a different market and marketshare than dominating the mainstream PC market as they've done.
MS has made a big play, and it may be enough to keep them in the game and revitalize their brand image. Time will tell.