Working effectively in a monorepo does require suitable tooling (full disclosure: I am a core contributor to Pants, a monorepo build system with, currently, a Python focus). But with that tooling in place a monorepo can make it a lot easier to collaborate, to manage changes and dependencies, and to avoid balkanization and fiefs. Granted there are no silver bullets, but we are working to make tooling good enough to avoid needing the large in-house support team you allude to.
Having a large number of small repos can work if they don't have a lot of interdependencies, and maybe this is what you mean by "totally unrelated projects". But if there are significant interdependencies (and repos usually end up this way, in my experience, which may or may not be representative) things become very brittle, and it's very hard to make changes and reason about how they affect downstream code, not to mention the challenge of version resolution, aka "dependency hell".
In the end, managing a large codebase, whether it follows a monorepo or multi-repo architecture, requires suitable tooling. On balance I think that monorepo is often a better choice, but YMMV for sure.