> As if when you are new to a module, you are slow because you don’t know anything, then once you have expertise, you are slow because you know everything and are helping others.

This suggests that the proper way to keep team productivity high is to have all team members working on the product since the beginning, and treat them well so that they don't quit and don't have to be replaced by new ones. Maybe even start with slightly more people on the project than necessary, so if a few of them quit during the project for unrelated reasons, you can still finish the project with the remaining ones.

Probably not going to happen, because this goes against maximizing short-term productivity at the beginning of the project. The short-term productivity is maximized by having the team as small as possible, and only worrying about problems after they happen.