You know who looked like a top 4 D and had success in the AHL too? Joakim Ryan. Or how about we stick to your claim of second line and AHL? Benn Ferreiro.
To be fair Robertson has been pretty incredible at each level.
He posted historic numbers as an 18 year old with 55 goals in 46 games along with 86 points in his draft +1. This was the highest goal per game in an O season since Lindros. He put up almost a ppg as a 19 year old in the AHL, and has been a ppg at 20 in the A while getting some NHL time.
That's significantly better than Ferreiro or Wiesblatt we're at that age... Wiesblatt is under a ppg in his d+2 in the WHL. I know he's on a bad team, but Robertson was in the A almost at a ppg.
That being said the trade makes little sense for both teams. San Jose should not be trading young prospects, and Leafs should not be giving up the better prospect, as well as the better depth forward.
I would rather make this simplified.
Travis Dermott + 2nd for Jacob Middleton + Nick Bonino