The benefits of monorepos (in my experience) are all people/organisation based e.g. it can be easier to enforce standards/processes among 100s-1000s of engineers with a monorepo, or it can be easier to manage/release a very large interdependent codebase/eco-system being worked on/coordinated between dozens of teams.

However, this linked post makes a great point, those benefits are all 'scale' problems which 99.99% of orgs don't have. The corollary is I've seen how hard it is to go from from multi-repo -> monorepo when you reach the scale where you would see some benefit.

I also think that the tooling/UX doesn't publicly exist to solve the multi-repo problem with 100s-1000s of engineers working on 100s of repos. It becomes so hard to navigate, understand and grok and so much is buried in dark corners. My experience is that that tooling is less hard to build around monorepos (Google for example).