More people means more human error, which means that not only do you have to invest more in communication overhead, but you have to invest more in sanity checks to keep your state machine in bounds.

The original author always knows not to push buttons in this order. Two people who are communicating badly will find a way to do so sooner or later, and if the system doesn't slap you on the wrist for doing so, it's gonna be a bad day.

On the plus side, systems with these sorts of guard rails are better opportunities for self-directed mid-level developers to pull themselves up to a senior role. One man bands are notoriously awful at affordances for discoverability.

I think I do better than most but any time I want to shed a responsibility I always have to first add more docs and a handful of extra exceptions and/or exception handlers to the code to make sure the new person doesn't immediately have a bad experience and toss it back to me. I call it sweetening the pot but others might use a different description.