Earlier quoted context omitted.
If your monorepo compiles to one binary on one host then fine, but what do you do when one webserver runs vN, another runs v(N-1), and half the DB cluster is stuck on v(N-17)? A monorepo only allows you to reason about the entire product as it should be. The details of how to migrate a live service atomically have little to do with how the codebase migrates atomically.
But isn't that a self-inflicted wound then? I mean is there some reason your devs decided not to fix the DB cluster? Or did management tell you "Eh, we have other things we want to prioritize this month/quarter/year?" This seems like simply not following the rules with having a monorepo, because the DB Cluster is not running the version in the repo.
Being 17 versions behind is an extreme example, but always having everything run the latest version in the repo is impossible, if only because deployments across nodes aren't perfectly synchronised.