So a monorepo makes it much harder and costly for library maintainers to break downstream dependencies, while in a typical environment they can break their API

So a monorepo makes it much harder and costly for library maintainers to break downstream dependencies, while in a typical environment they can break their API every new commit and create a ton of work for everyone else without a second thought. I worked on a microservices project where downstream projects put "contract tests" into upstream dependencies. Ie, you can change your dependency if you want, but it has to match the contract/API we agreed with you or your change doesn't go in.