We have exhausted this topic as well. Check the March 2017 archives. We talked in circles about how the show ruined Dar’s character with the implications he had assaulted underage boys in his group.
But I still think Carrie and Saul are much more compelling than Dar and Quinn ever were. There were of course parallels between their respective relationships/mentorships but Carrie and Saul’s relationship has always been heaps more interesting and complex to me. Obviously YMMV.
However, Dar and Quinn weren’t “phased out” for Carrie and Saul. The Carrie/Saul relationship is the central relationship on this show, from the beginning. Probably not the answer you wanted to hear.