Contrary to less, PowerShell is definitely not backwards-compatible with cmd. Microsoft has changed cmd a select few times, most notably adding command extensions around NT 4, I think. But those changes were very careful not to break existing scripts (which means delayed expansion is always disabled by default). If vi acts a little bit different, or more accepts other keys that don't scroll a whole page, it's also not a big deal because those are tools people interact with, not programs. People are a bit more flexible with change. And Microsoft certainly changed a lot of things in our around its user interfaces.
cmd is still around for the same reason sh is still around. For compatibility.
I disagree; would it be Unix/Linux without a bourne/csh/bash/whatever shell? Would Win3.x/NT/Win10 be the same without CMD.EXE? I have built, developed in both, heavily, and I say that Unix/Linux without a shell is not really Unix. But the Windows without a shell may disadvantage admins, but wouldn't be missed by the vat majority of users.
cmd is still around for the same reason sh is still around. For compatibility.