That makes sense from an engineering perspective, but from a business perspective you don't want the understanding to live in the heads of a small team of people. Companies own the codebase, they don't own their employees. If losing a single employee means losing the understanding for a significant chunk of the codebase, that's a serious risk. With the senior/junior model where you have a senior engineer architecting the system and a small team implementing that architecture, the understanding lives in the senior engineer's head, but it also lives in the head of the person who implemented it, and their teammates who implemented the parts that interact with it and were present for the discussions probably have enough knowledge to figure it out pretty quickly, so as long as the company doesn't lose the whole team all at once they're okay. If you make the team more productive with AI tools you can have the team do more, which still cuts down the total headcount, if to a lesser degree, but preserves the redundant understanding.