Somewhat related. I built an Elm app all in one file as an experiment and to see if I like it. It's a little over 7k lines and I'm occasionally adding more to it.
It's actually pretty straightforward if you're in a language with lexical scoping, and it simplifies some things, like includes / cyclical, no modules, no hunting through files, etc.
I feel like this set up could integrate really well w/ AI models.
I've found that the only real limitation, at least in my experiment, was a lack of decent editor support. I use vim so this wasn't really much of an issue for me with many great ways to navigate a file, and a combination of vertical and horizontal splits on a large screen, but when I opened it up in other "modern" editors the ergonomics fell apart quite a bit.
I think the biggest downside was re-using variable names between large scopes occasionally made it hard to find the reference I wanted (E.g. i, x, key, val), but again, better editor support allowing you to limit your search to within the current scope would help. Also easily mitigated with more verbose throwaway variable naming.