This part about "a lot of die space" is largely false. The die shots show that AVX-512 adds only a modest amount of area, mostly for the wider register file and

This part about "a lot of die space" is largely false. The die shots show that AVX-512 adds only a modest amount of area, mostly for the wider register file and 64-byte path to memory. The "huge power hungry" execution units are in fact largely the same execution units used for AVX and AVX2 - the CPU simply uses 2 adjacent 256-bit AVX/2 units as a single AVX-512 unit: this is the so-called port 0+1 fusion. Only the larger shuffles and the (optional) extra FMA unit on port 5 take significant addition space. In fact, the shared core die area for SKL (no AVX-512) and SKX (has AVX-512) is the same! There is some dead space on the SKL die due to lack of SKL support but it is small: you can bet Intel didn't ship 100s of millions of SKL and SKL-derivative chips with a big space-wasting AVX-512 unit which couldn't be used on it: they could have made a new, more targeted layout if that was the case.