The benefits of monorepos are (IME) all in people and organisation scale. For example, I've found it orders of magnitude easier to align tooling, quality, release and standards on a monorepo with 100's of 1000's of engineers working on it simultaneously.

The point you've made is a good one though, the idea of monorepos at small scale has come up a lot recently and I'm not sure the arguments for it are as compelling (I don't see many compelling arguments _against_ it either mind you). What I _have_ seen though is the pain large orgs go through going from multi-to-mono repos, I think Twitter did it recently and it was pretty painful.