There is so many low hanging fruit in software design that is simply there because of legacy design trade-offs and the cost associated with replacing those - we could gain huge performance gains over night if we for eg. eliminated reliance on hardware memory protection and context switching from kernel space to user space by using languages that can prove memory safety in software.

Then imagine how much performance you could get out of OS level VMs that understand the processes at VM level (ie. can access code in some IR that they can analyze easily, recompile it on the fly, etc.) there is already stuff like this in specialized markets (eg. kernel level GC for JVM) but it's still fairly specific.

Then there's all the shitty legacy abstraction layers in things like filesystems - ZFS is a perfect example of what kind of gains you can get for free if you just rethink the design decisions behind current stack and see what applies and what doesn't.

If the benefit of rewriting these systems ever overcomes the cost - we have huge potential areas for performance gains, modern systems are very far from being performance efficient, they are efficient based on various other factors (development cost, compatibility, etc.)