You posit what amounts to the worst case scenario. Sure, if the team still stinks in seven years then yes, you have to consider trading Hischier. If the team continues to be this bad for the next seven years the team would owe it to him to trade him to a better team. He'd have PTSD by then.
I think an alternative is that the team's draft picks start panning out. Younger and/or unproven players like Hughes, Smith, Boquist, Anderson, and this year's first round picks develop and are better, in some cases significantly than the players they replace and the team becomes good. Maybe not immediate cup contention good but at least moving in that direction. At that point, you can decide who is in your core and pay accordingly. A 28 year old Nico on a good team coming off a couple of solid playoff runs where he made a difference is a good signing even if it takes him into his 30s (assuming he is healthy when he signs). He's a heart and should guy and a hard worker and sets a good example based on publicly available media. Hopefully Hughes grows into that as well and it's easy to advocate for a long term signing for him at that point.
It just seems premature to me right now. There's no real defined core in place. Is MBW a fluke? Will Hughes break out? Will Nico develop more of an offensive flair? Will Smith replace Butcher and upgrade the second pair? Lots of questions that need to at least suggest answers before I'd be comfortable trading good assets for a borderline core player at 28-29 to a hefty 8 year deal (if OEL is the example). You make a good point, but the counterpoint, to me, isn't as black and white as your example suggests. Just my opinion, not a fact.