1) I don't want shell to be full-blown high level programming language. POSIX, Bash scripts/languages are enough for my shell uses.
2) For anything high level or for tasks shell would become complex, We do have dozens of fully matured languages. I would rather use those instead of this/any shell script.
3) Features desirable for shell include syntax highlighting, autosuggestions,command-not-found helper,tab autocomplete, flag complete. My shell already has it.
4) I like bash and zsh as shell. Bash is simple,i like it,but it doesn't have few features that i mentioned. So,I don't use bash as my main shell. zsh also is compatible with most bash/POSIX and it has all tte features that i mentioned in (3).fish also has these features but it's syntax is different which i don't tend to use. So,i don't use fish. zsh has all features that i like in fish too(mentioned in (3)) .