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Many years ago I worked with a greybeard AMD hardware designer. He told me that they commissioned a study about whether it made sense to ditch backwards compatibility, and realized that the parts of the CPU needed to support backwards compatibility they were willing to ditch contributed to less than 1% of die area, and of course were all already designed and battle tested.

Unfortunately I don't have a better source for this than an anecdote.

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You could lose the entire x86-specific part of the die in a corner of a 512x512 FMAC unit. Die cost due to x86 complexity seemed like something worth attacking in 1990 when RISC was gaining mindshare but over the years the complexity of that has not expanded very much while the other stuff on the die has gotten much larger.

That probably depends on whether you mean old instructions now cause a fault or moving x86 to a new format entirely that doesn't need a decoder from hell

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