In the post yesterday one of the arguments was that if nobody checks out all of the code then what's the value of having the code all in one place?

Last monorepo I worked on, individual contributors checked out just the tree they were working on (we had a suite of applications with several shared modules). We made it simple and straightforward for them to get what they wanted and ignore people whose work didn't impact them.

But the senior people, who were better with architecture and version control trivia, checked out the entire thing. They would steward any cross-cutting changes that needed to be done, and make sure any callers to shared libraries were updated in the face of breaking changes. They were also backstopped by the build plans, (some of) which also checked out the entire thing.