I think you're better off having a spare machine and your data in the cloud than to have 2 different CPUs on the same machine. It would massively increase the price and complexity, reduce reliability, etc. It's 2 machines in one box somehow. In the end your machine will not serve its purpose with the backup CPU or you would have bought one with that as a main CPU from the start.

We're going in the direction where the hardware is a commodity to replace as you see fit and the data stays somewhere safe to be used regardless of device. Phone, tablet, PC, console, etc. can consume the same data.

It's not ideal because I'd like hardware to be reliable, not something I can expect to die on me when I need it most but it's pretty clear we're going that way outside of niches, with most devices being exceedingly hard or impossible to upgrade or repair.

My way of thinking was about high load servers mostly, where your idea works well for personal computing. In both cases a whole computing system should be designed to allow easy replacement of depleted silicon. Nowadays electronics designed in such a way that maintainance either extremely difficult (for an average consummer) or not possible at all, both technically and legally.