> When I say maintainability, I mean the specific thing where it becomes really, really hard to change one part of the codebase without breaking another part.

The corollary of agents being bad at maintainability but good at coding is that you can vibecode all the parts where maintainability doesn't matter.

So if you build a (domain-specific) modular architecture for your software first you can then just let your software factories loose on building the modules.