I think a couple things about Rapp.
1) He missed most of camp due to injury and that set him back.
2) Fuller has been a nice surprise. He has great size for a Safety, is physical, and is sound in his protections. But... he was considered a tad slow coming out, like Kupp and Jefferson (when will the NFL ever learn). But I think Staley loves his size, smarts and ability to deal with bigger receivers and to tackle with a thud.
3) With JJ back, Rapp was never going to get his spot.

I don't think Rapp is a disappointment. I think he's a victim of system and D philosophy, and that Fuller just may turn out to be a heck of a Safety. Next year will be a different story with JJ moving on. Clearly they like to play Fuller deep. Rapp is better playing up.