The only problem with your scenario is that Trouba won't sign an offer sheet, so there is no "value cap" as you state.
I'm not sure it's a problem with my scenario as much as it is with perception.
Connor and Laine are both due big deals. Laine hurt his value, but the two of them together almost assuredly end up eating half of that cap space.
Now you need 3D, 4F, 1G and you have 13.5m in space to make that work. Figure you'll bring up a rookie or two, lock up Copp for 2-2.5ish - but that's a lot of dumpster-diving to fill the roster if you're going to bring back Trouba post-arbitration - that's yet another wildcard.
So if I'm a team that wants Trouba - lets see what I know:
-Winnipeg's probably got a cap issue that precludes them from paying him his market value without making a trade
-Winnipeg's probably got to take him to arbitration
-There's been a lot of smoke around Trouba wanting out over the years - it's ebbed and flowed
-There's a pretty good chance he's a 2020 UFA if he stays in the 'peg
He's locked in value-wise by Winnipeg's cap situation, and it will get worse for Winnipeg over time unless something gives. And look, it CAN give - Perreault, Little, Lowry, and Byfuglien are all probably at least for discussion in trade once this season ends - but as things stand, why would any team give up more than offer sheet value for him knowing where Winnipeg stands?