The problem is not doing anything serious. Of course this would need an updated language runtime.
The problem is doing something simple, like an installer. bash => zsh: fine. You can still install with the old /bin/bash. but for many cases a shell is not enough, and then you have to compile something statically for a trivial dynamic task. Which makes the download 10x larger. Not cool.
The standard toolkit for this is perl. It's used in automake, most gnu utils and git. python and ruby bring nothing new to the table, besides being newbie friendlier.
The problem is doing something simple, like an installer. bash => zsh: fine. You can still install with the old /bin/bash. but for many cases a shell is not enough, and then you have to compile something statically for a trivial dynamic task. Which makes the download 10x larger. Not cool.