It's not as big of a haul as it looks
1) Minn has had years to monitor Buium, they're first rate at evaluating and developing talent, and he's been awful defensively this season and cooled off after a hot start offensively. They have to be frustrated by his inability to play defense - this isn't Drysdale being ruined by Anaheim. Was demoted to the 3rd pair and 2nd PP unit. He's not the elite prospect he's hyped to be (compare to Lane Hutson at 20). He may turn out to be a top 4 D-man down the road, but they're selling high. If they really thought he'd turn out to be elite, no way they trade him for a rental.
2) Rossi is a good scorer but is limited due to size (5'9 182) and only slightly better than average speed. At 24, not a lot of upside left. Minn demoted him to 4th line in last year's POs. He's a regular season stat padder, not because he doesn't play a team game, but in tight checking PO games, his physical limitations get exposed.
3) The first is likely to be 25+.
Minnesota has a deep prospect pool, a young team, and the ability to reload if Hughes leaves and trade to replace him if need be. They're trading from strength and were a top 8 team before the deal, so it's an educated gamble, they get two PO shots during his peak years and have enough organizational depth to move on - they didn't give up core assets.