> Bugs can and will happen, eg. when A depends on B and some developer accidentally makes a change to B that will pass B's tests but only break in A's use case. In monorepo case such bug can be noticed before the broken code hits master.

I don't see what point you tried to make. So A breaks when someone breaks B. That's ok. You spot the error the moment you try to update A, well before you commit any code. You file a low-priority ticket and go on with your life because A is still good in production.

> Knowing when no one uses older versions is also hard (...)

Not really, it's the easiest part of the whole thing and a non-issue. If you manage a package this does not even register as a problem. If it's a service you have request metrics.

What is the problem, really?