> Also, in multicore CPUs, there are generally some spare cores that will get swapped in if a previously in-use core breaks.

That sounds like a huge cost to bear. Looking at e.g. a Haswell die photo [1], there are just four physical cores present for a four-core part. With that die area per core, you would take a ~15-20% area hit (that translates to 15-20% cost) just to have a spare core in case one failed some years later.

I have heard of manufacturers selling otherwise "defective" parts where a core or cache slice has a defect by relabeling as a part with fewer cores. But that's a manufacture-time decision, not a dynamic reconfiguration in the field.

[1] http://cdn2.wccftech.com/wp-content/uploads/2013/05/Intel-Ha...