This is what always scared me about monorepos. While I'll never reach the scale of big tech it was always a nagging feeling that I'd hit some limit. I know that is less likely to be a problem with smaller component based repos and if something goes wrong only that component is impacted.

They would be hitting different problems. Taken to a ridiculous opposite extreme, think about your most recent project. Think about having it organized into a separate git repo for every file. One file per git repo. Absolutely ridiculous. You'd need a bunch of glue programs to deal with the pile of git repos you now have. How do you handle the problem of different branches for every file? You need some sort of orchestrator. It's a thought experiment, you wouldn't actually do that, but at scale, eg Google, Facebook, and now OpenAI; that's where they operate. Orchestrating between a bunch of different repos is a different problem entirely but a monorepo solves a lot of problems while creating others. Some are more solvable than others.