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.
SDL1 can be installed alongside of SDL2. Same with GTK and Qt. I really fail to see the issue here. If a developer of a Qt4 application doesn't feel like updating to Qt5, users can still use the Qt4 version with Qt5 installed in their system. Most Linux distributions still package Qt5, Qt4, Gtk3, Gtk2, Gtk1.2, SDL1.2, SDL2...
On top of that, if a binary-only application really wanted to stand the test of time, it could statically link the libraries.
- https://github.com/sn-00-x/aa4mg