Legacy instructions and compatibility overhead is always mentioned when talking about AMD/Intel yet its completely irrelivant as that's their selling point rather than a flaw for other companies to take advantage of. If ARM/mips/VLIW arches or companies manage to gain marketshare in the server industry then that means the server guys have less dependency over these very same legacy ISA's and it would be trivial to get rid of them.
To me it seems irrelivant wether we pay 10% overhead for x86 ISA's or 0% for an ISA that can't replace x86.