The article goes into it a tiny bit, but the cost is the mental cost of when you do need to work on it, understanding it, and I would add keeping the tooling the same.
Encouraging devs to have their changes include all modules, even those that are old and mature and don't need to be touched, is a good way of ensuring this doesn't build up to where it becomes a problem.
Encouraging devs to have their changes include all modules, even those that are old and mature and don't need to be touched, is a good way of ensuring this doesn't build up to where it becomes a problem.