Interesting but it would be pretty rough to implement. If you take a binary now and run it on a core without the correct instructions, it will SIGILL and probably crash. So you have these options:

Create a new compilation target

- You'll probably just end up running a lot of current x86 code exclusively on performance cores to a net loss. This is how RISC-V deals with optional extensions.

Emulate

- This already happens for some instructions but, like above, could quickly negate the benefits

Ask for permission

- This is what AVX code does now, the onus is on the programmer to check if the optional instructions can be used. But you can't have many dropped instructions and expect anybody to use it.

Ask for forgiveness

- Run the code anyway and catch illegal instruction exceptions/signals, then move to a performance core. This would take some deep kernel surgery for support. If this happens remotely often it will stall everything and make your system hate you.

The last one raises the question: which instructions are we considering 'legacy'? You won't get far in an x86 binary before running into an instruction operating on memory that, in a RISC ISA, would mean first a load instruction, then the operation, then a store. Surely we can't drop those.