And you can do that in a monorepo. But realistically, there are plenty of trivial breaking changes (renaming Foo to FooX) that don't warrant that effort, and so usually don't get done outside of monorepos.

You should apply care when making breaking changes. Having it be hard is a separate issue - I'd say distractions from multi repo tooling would introduce more risks overall. Having a unified CI system in a monorepo is really nice.