If the questions asked on Unix and Linux Stack Exchange are anything to go by, the Z shell already has mainstream adoption. But of course your argument has the

If the questions asked on Unix and Linux Stack Exchange are anything to go by, the Z shell already has mainstream adoption. But of course your argument has the fundamental flaw that the Bourne Again, Korn, and even Bourne shells were not compatible with their various predecessors, but that turned out to be not as problematic in practice as you paint it to be. And we've had all sorts of things gaining "real adoption" over the years, from Norton Commander clones to Perl. The world is nowhere near as narrow as you think.