I could not disagree more.
You can scope an agent to a folder. You can arrange a monorepo any way you like. Every team without a monorepo inevitably builds some "fetch all my repos and provide them to an agent as context" and then has to open 10 pull requests that must be merged in some special sequence.
If you cannot make modular monoliths and monorepos work, please trust me: it is a skill issue. There are just things you don't know and you should go learn them.