I think some of the reason Trent is able to be so effective is because he has the room to hit those raking passes in the position. I don't mean to say that in midfield he wouldn't do it, I don't know - but playing someone who can play passes like that at full back is the entire point. If you think about it, Trent having the ball pulls the other team towards his flank, because they have to cover that position because of his crossing, because of Salah, and so on. That in itself makes the switch that much more dangerous because Robertson has that much more room on the opposite flank. Trent is the playmaker from that position, and the midfield to a degree are there to allow him to do that - Gigi keeps the ball and sucks people in, Henderson makes runs down that flank to cause extra problems, and Fabinho covers for the other two. None of them are Alonso's but it's better that they aren't - because Trent not only does that role anyway but is given more room to do so because of the nature of the fullback role.
I do kind of wonder if a team is going to play a competitive midfielder on that flank at some point, just to shut him down. It might prove to be an effective tactic.