If Mantha can stop breaking himself with freak fighting injuries, he's a proper first line winger. The wings are at rock bottom, now but will only get better - their vets are their worst players and the bad contracts are mostly ending or nearing an end, and even if the likes of Zadina, Rasmussen and others don't hit the hoped for level when drafted, they are already better than about half the current roster.
And I guess knowing a top 4 pick and likely another year or two of lottery picks are on the way makes things look more attractive long term, and especially as Mantha is good mates with Larkin & Bertuzzi, whom he has great on ice chemistry with