I've worked at a shop with a lot of legacy code and a very fragile build environment. The myriad of scripts that source scripts that source other scripts to produce a working build were all written in KSH - as well as the majority of the tools to set up an environment to actually run the software. Since these scripts were intended to be sourced, adding a shebang doesn't solve the issue at all.
Fortunately, zsh can operate in ksh emulation mode [1] so I was able to leverage zsh while I was there to some extent.
The system I worked on there is still being actively used and developed today and still relies on a fragile set of ksh scripts to build and run in certain environments.
- https://github.com/sn-00-x/aa4mg