> In my dream world, every engineer can and should be allowed to intervene in as many layers/slices of the code as his mental capacity and understanding allows.

The problem is that a sufficiently-complex system is impossible for a single engineer to comprehend in its entirety, so each engineer will end up only understanding a subset of the overall system. Splitting a monolith into clearly-delineated services - and splitting those engineers into teams developing and maintaining those services - helps make that understanding less of a daunting task.

I agree that engineers should be able to move between these different concerns mostly freely (so long as there are indeed engineers covering all concerns), and should be encouraged to learn about those components which interest them, but that clear delineation and a clear notion of "ownership" of different components expedites the process of figuring out who can fix what (since "everyone can fix everything" is unrealistic for all but the most trivial systems), who to call first when something goes wrong, who should be reviewing PRs for a given component, etc.