The thing to me is always that, yeah, you need a huge commit for a breaking change in an internal library inside a monorepo, but you will still need to do the same work in a polyrepo eventually, too.

Especially since "eventually" really means "ASAP" here. Without going through the breaking change, you can't benefit from non-breaking changes either and the complexity of your codebase increases the longer you defer the upgrade, because different parts of your application have different behavior then. So, even in a polyrepo, you ideally upgrade all library consumers right away, like you're forced to in a monorepo.