I don't know, I haven't felt like there was much chemistry between DeBrusk and Pasta. I know they haven't played together much but I think that's what Cassidy sees when he tries it. Pasta plays his best off the tic tac quick passing plays and DeBrusk really doesn't pass the puck, at least not last year. When he got the puck, he'd hold it and try to get to the net on his own.
Heinen also didn't look like he could hit the broad side of a barn in the playoffs. His overall play and playmaking were good, but he seemed really snakebit shooting the puck. Not sure they wanted that on the first line in the finals.
That said, I'd be all for putting Heinen there next year. Let him grow into the role of a first liner. Maybe the success would give him more confidence in the playoffs. I'm also all for trying Pasta with Krejci in the regular season, I'm just not convinced DeBrusk can play with Pasta. I wish they'd consider playing Jake on the 3rd line with Coyle but it feels like the 2nd line is more DeBrusk's than Krejci's line now, I can't see them taking Jake off it.