you don't write shell for performance and legibility it's for portability across all legacy crud and it's an honest job because sometimes you just want the thing to run on every mystery router in existence without depending on a particular interpreter. you know literally the reason for existence of posix
otherwise I agree everything is better tailor fitted for the os which is what a {linux,freebsd,openbsd,netbsd,dragonflybsd,illumos} distribution does, not gnu because gnu doesn't ship operating systems
also csh is not a superset of posix sh jackass
