I think an organization needs to have confidence in their assessment of their top prospects, even before they've been tested in the NHL. Consider Samberg, who is now performing as a core anchor on the Jets' D. He was projected to be a shutdown D from his college days, and that's what he is. I think Salomonsson has the same potential to be a top-4 D with more puck-moving ability. If so, he's a very valuable asset, partly because he's a RHD and partly because he'll be inexpensive for his first few seasons.
That said, if the Jets are convinced that Cozens can rebound to be a bona fide 2C with offensive impact, they might justify the cost. But I'd bet they'd look at trying to move other assets before Salomonsson. The Sabres might want a young RHD but I doubt they'll find a team willing to trade a top-end RHD prospect.