These are not six independent products bolted together. They are one path, and the order matters because each step constrains the next.
Source selection decides what can ever be retrieved, which makes it the highest leverage decision on the list and the one most often made in the first ten minutes without thought. Retrieval decides which passages reach the model. The model phrases an answer from those passages or says it cannot, which is grounding doing its job rather than the model being weak. Where it should not guess, escalation moves the conversation to a person with the thread intact. And every refusal is recorded, which is what closes the loop back to sources.
The practical consequence is that upgrading one part rarely fixes a problem caused by another. A better model does not repair a bad source list. More sources do not improve an answer that failed because the right passage was ranked fourth.