Semedo doesn't offer just pace, that's gross exaggeration. Semedo is a very good dribbler, he is strong, agressive, very very hard to beat in 1 vs 1, can cover a very large area in defense basically allowing our two CBs to drift towards the left where Alba goes up very very high all the time. And that athleticism (pace, power, speed) you talk lightly about are key attributes in a fullback. The ability to hold your own against your direct opponent, sometimes even against two, somtimes going in the center and stoping counters when our CBs get pulled around or overran by faster players, those are basically the bread and butter of a top RB most of the time.
You thick them at 'just pace'.