It's easy to dismiss Kiwior when he's behind the best centre back partnership in the league. If Gabriel and Saliba hadn't settled into such a robust and well-functioning pairing, Kiwior's experience may have been much different. He's personally suffered significantly from one of the truly great aspects we've benefitted from in the last year or so.
It's a difficult call with the third/fourth choice centre back behind such a strong pair. Do you go with developing youth who have mistakes in them (Kiwior), average but happy to settle (Holding) or seasoned professionals who may cost a packet? When you've got squad players who can rotate like White and Tomiyasu I'd take the chance on players like Kiwior all day long.