I think the question is will Trouba ever sign long term here? If he will then this offseason is the time to lock him up. If he won't, to me it depends on what the offers are right now vs what you would get next offseason with only 1 year of control remaining. You'd be talking a late 1st + roster player at the trade deadline as a pure rental so presumably you could get that next offseason when the acquiring team has a full year of Trouba. Maybe even with a conditional pick based on if he signs an extension or not. Are we able to get something significantly better than that now?
Going for the cup one more year with Trouba is acceptable to me but I'd know that he's just a Merc and will be gone next offseason.
Trading him would be a big problem as we'd critically need a top pair potential RHD prospect or 2 in the pipeline to replace Buff and potentially Myers if he can't be extended. rolling Buff, Myers, Poolman on the Right side is still respectable to me. Not as good as if Trouba stays, but if we can pull Trouba + Petan for both Detroit's 1sts (6th and 30th), Ottawa's 2nd and Hronek. I could get behind it. At 6 you would guarantee you'd get one of Dahlin, Svech, Zadina, Dobson, Bouchard, Boqvist or Kotkaneimi (If the Jets see him as a long term option down the middle behind Scheif). Then at 30 and 33 grab another RHD and/or a Center prospect.
Suddenly you're Buff, Myers, Poolman, Dobson, Hronek, (Woo? Lundkvist? someone else) and the D prospect pool on both sides looks strong going forward. We just need to hit on someone to get the top pair RHD we need in 3 years time to replace Buff.