I think it is not really the execution units for simple instructions that take up much chip area on application-class CPUs these days, but everything around them.

I think support in the OS/runtime environment* would be more interesting for chips where some cores have larger execution units such as those for vector and matmul units. Especially for embedded / low power systems.

Maybe x87/MMX could be dropped though.

*. BTW. If you want to find research papers on the topic, a good search term is "partial-ISA migration".