zsh is not a "cleaned-up bash"; it's more of a clone of ksh (closed source at the time), with some csh features added in, as well as their own inventions. bash and zsh appeared at roughly the same time, many features were added in zsh first and added to bash later (sometimes much later, and often never).
This is kind of a good example of what I meant when people conflate "bash" with "shell".
As for your larger point: I kind of agree, but I think what zsh offers is the advantages of shell scripts with compatibility with existing scripts while still improving on it. That said, I believe oil also offers compatibility, but I haven't had the chance to look deeply in to it; just haven't had the time, and wanted to wait until it's stable (maybe it is now?)
Perl was initially invented as the "modern glue language" to replace shell. It's fallen a bit out of fashion these days though, and to be honest I never cared all that much for Perl myself either. Raku looks nice though. TCL also works well as a kind of "glue language", although it has some really odd behaviour at times due to everything being a string and I know some people hate it with a passion, but it always worked fairly well for me. But that has also fallen out of fashion.
I've also been told PowerShell is actually quite nice and has interesting concepts (and now also open source, and you can run it on e.g. Linux), but I could never get over the verbosity of it all. I'm an old unix greybeard and I want my obscure abbreviations dammit!
