For the record, I've had successful FWB relationships that started with the benefits and moved to friendship. My friendship with Harry was like that. We met at a kinky play party and ended up having (oral) sex that very night. A week later we met at his place and hooked up, then went out for food afterwards. We've always had a very sexually charged friendship, but it also really did become a friendship -- he lent me some great books, I helped him move, we watched Game of Thrones together, etc. When I started dating Davis, I told Harry that we had to take a break, and he was disappointed, but we've remained friends for the last year even though the benefits have been out of the picture. In the meantime, Harry formalized his relationship with the girl he was sort of dating (it was all above board, she's poly and she knew about me), and she moved in with him. I went to their housewarming party over the summer, brought my roommates along, it was a great time.
Just wanted to point out that it can indeed work that way.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.