In the end, this is the summer to make a trade for a dman. Players will be available due to expansion draft next summer. Teams will be looking to get something for their players while they can. Trouba is the perfect target.
If you think this class is weak, wait until next year. Lots of 35+ players contracts expiring and not much else. Hedman will be a UFA next year, but I can't imagine he'll make it that far especially if Stamkos does leave TB.
In today's NHL, who would be willing to part with a top pairing defenseman? I agree that we need a stud d-man, but who are you thinking about getting that anyone would be willing to part with? PK Subban? Shea Weber? Ryan Suter? Kris Letang? Alex Pieterangelo? Drew Doughty? Erik Karlsson? Those are some of the best in the league right now and I can't imagine any of them moving even if we offered Dylan Larkin. Even if we were able to swing a deal for one of those guys or some other top tier d-man, that's not really going to fix the problem. The unit as a whole isn't very good. The UFA defensemen aren't great, but they're better than what we currently have and I think several minor upgrades would be in our best interest considering the unavailability of top tier d-men around the league.
Trouba might be available. The Jets have 3 defensemen with a nmc, they'll either have to expose Trouba or only protect 4 forwards as it stands now, assuming the PA gets their way and players with a nmc have to be protected. Unless they make a deal with the expansion team(s) they'd lose someone they really don't want to lose. Trading Trouba or convincing one of the others to waive their clause is their best option.