Unless the physicists find some way to breathe new life into Moore's law, I suspect we will gradually move back to the mainframe approach of implementing more and more of the low-level stuff directly in hardware.
Power series is a possible contender, but then you see the glory that is IBM behind it, and you know that no one wants to deal with that on the larger time scale (swap Intel for IBM? Why do this?)
Sparc is all but dead. MIPS is effectively dead. I've heard good things on RISC-V, though the question is, who will want to produce a non-differentiated CPU, when others can do this as well ... that is, you can't really extend RISC-V unless you break the ISA.
Then there are the toolchain issues.
Having experienced the Calxeda failure as a partner, realizing that the ARM marketing claims of low power Intel replacement were complete nonsense[1], I am not all that interested in climbing back on that particular heavily hyped horse.
[1] https://scalability.org/2013/12/the-evolving-market-for-hpc-... search for ARM.
ARM32 has "ARM hell" with multiple extensions but they've mostly fixed this in ARM64. You can definitely have compatible ABIs at the level users care about, namely application binaries.
Multiple vendors means price competition too.
IMHO the major stumbling block for ARM64 vs. X64 is that X64 has so much existing market share. Installed user-base is very powerful and everyone knows X64 will work so why take the chance? Hardware is cheaper than IT person-hours.
If someone fielded an X64-competitive ARM64 multi-core chip at a competitive price point it would probably get some traction.