So in Dani Alves peak years, most of which came in a Barca side that had 70% possession and were winning by cricket scores each week, he assisted at a lower rate than Trent? For balance, at a similar stage in Alves career to what Trent is at now (I've not gone in to too much detail, just his first 5 seasons), he'd picked up 11 assists in all comps. Trent has 40 odd.
The fact that Trent's assisting at a rate better than any of the previous great attcking fullbacks despite not reaching is peak, isn't what prompted me to say he's redefined the position - as fez said, we've always had attacking fullbacks, just not as effective as Trent. What Trent has changed is that he's not just another Alves, Marcelo, Evra, Cole or Andy Robertson or whoever else that bombs down the flank, overlapping the wide player. Of course, he does that too but that's not the limit to his attacking contribution. He gets on the ball in far deeper positions and influences the game for 90 minutes, not just 5 minutes when he's making overlapping runs. Look at his performance vs Utd at Old Trafford for example, how many of the chances he created or attacks he started came from passes closer to the halfway line than goalline. And that's one example of many. In Liverpool's two best seasons in my life time, he was the main playmaker doing the things Gerrard would have done in years gone by or what KdB does at City.
The only other time (at a high level anyway) I can remember fullbacks having the same level of influence on games that I can remember was during Pep's time at Bayern - like Trent, Lahm (when he wasn't playing CM) and Alaba where playing as midfielders in possession, controlling the game but they didn't have anything like the attacking output Trent has.