In the old days this would be no biggie as every non-trivial site would have at least half a dozen different kinds of Unix workstations anyway. SPARC, MIPS, PA-RISC, AXP, m68k, POWER...
In the good old days everybody just used HTTP, FTP and telnet and NIS. Old archs also has other bugs we forgot about / never discovered.
The point being: monocultures are fragile.
Spectre impacts current iterations of: x86, ARM and Power. It's most likely impacts other archs that have variants of CPUs with out-of-order execution.
Spectre is harder to exploit - it’s the Intel-specific one that’s the biggie.
You think it'd be less work having to coordinate with even more vendors than listed in the advisory?