That seems like a disaster brewing. The whole spectre family of vulnerabilities is a side effect of CPUs keeping state around to optimize things and leaking that data between privilege levels.

I mean, in the humorous extreme: imagine if some enterprising group at AMD got together and realized they could "optimize" all that retpoline code by making the RET instruction aware of the branch prediction cache!

"Fundamental architecture changes" are, in fact, what is actually required here.