I agree as well. Especially in the context of recommendation systems, the decoupling of retrieval from a heavy ranker has a lot of benefits. It allows for 1) faster experimentation, and 2) the use of different retrieval sources. In reality, the retrieval might consist of a healthy mix of different algorithms (collaborative filtering, personalized page rank, word2vec/2tower embeddings, popular items near the user, etc.) and fallback heuristics