What about some sort of complexity checking more precise than lines of code? What about trying to write one thing you would normally write in shell with something else per unit of time.

I would simply say if you're planning on making changes to the script you probably shouldn't write it in bash. Or to be a little more rigorous, don't use bash if you couldn't rewrite the entire script, correctly, from scratch, in an hour.

Although I think all of these things are just complicated ways of saying "please seriously reconsider writing it in bash."

But I write bash scripts all the time, I just try to keep them as short and simple as possible.

Will, this is more about giving someone some quick guidance in passing, that they can apply without too much, um, complexity.

As someone who likes small fonts and big screens, for me a screen is often a lot more than 100 lines.

I don't think this is like the 80 character line length that's about screen size. This 100 line limit is framed as a quick and dirty for heuristic for script complexity.