> Counter-argument: Just let it sit there and keep working? Outside of security patches, if there even are any, there's always the option of "just stop touching it."
That's not exactly possible with a monorepo that has no branches and everything has to be maintained 'in step'. Any time a library breaks API, all dependents must pass tests, and/or be fixed so that they do. Any time a runtime API breaks (think things like interfaces to runtime authentication, compute scheduling, database services, network bandwidth scheduling), updated services must be made to conform and then be rolled out. As with real life, this might mean getting a 1 or 2 year old deployment system (configuration for production, roll out code, ...) undusted, understood, probably fixed in turn, etc.
As such, some SRE/SWE team _must_ be responsible for projects that are in the "don't touch"/maintenance phase. This is very unsexy, toil-y and you don't get promoted for it - and as such barely anyone wants to do it at Google.