This kind of mitigation really only makes sense on shared machines (such as servers). On a desktop OS, if an attacker is in a position to read memory from other

This kind of mitigation really only makes sense on shared machines (such as servers). On a desktop OS, if an attacker is in a position to read memory from other processes, it's pretty much game over already. Browsers implement Spectre/Meltdown mitigation on desktop OSes because without that, JS could read secrets from other JS contexts executing in the same process. One of the mitigations is in fact to just segregate JS contexts into different processes depending on the domain they belong to. But most apps don't execute untrusted code so most apps don't have this sort of in-process attacker to worry about.