The key point is that Icelandair will not be needing any "combos" of different airplanes if they go for the A321 (including the LR and XLR). That single type can fulfill every one of their requirements with the added bonus of more simplicity and cost savings that only having one airplane type brings.
No doubt that the A321's are great planes, but they can't fulfill every requirement that FI have. They need something bigger, such as the A330Neo or 787 too. They need them for slot restricted airports such as LHR and AMS and for the larger cargo capacity on some routes.
Types flown: A319, A320, A32N, A321, A332, A333, A343, AT43, AT75, AT76, B717, B732, B735, B736, B737, B738, B752, B753, CRJ9, DC10, DH4D, DHC3, E135, E145, E175, E190, E195, F100, MD11, MD81, MD82, MD87, RJ1H