I think the big question in improving the defence isn't who would be considered expendable, but who's available. I think there were about six dmen, who'd arguably improve our top 4 at a cap hit we could hit, who moved last year. There's no guarantee we have the sort of pieces that this year's sellers will want.
As such, I'm skeptical about the idea that trading Letang and replacing him with two 4m dmen will work, because I'm skeptical we'll be able to trade for two strong top 4 dmen at 4m cap hits. I can see packaging Maatta (or another dman) with something nice for an upgrade, because Klefbom and Brodie could maybe got in deals like that if that's what the other side wants, but getting two for Letang? Ambitious. Not gonna rule it out after seeing Rutherford get Brassard, but not betting on it.
Actually, maybe the question is who's expendable other than dmen. Do you move Sprong for an upgrade on the D? Jarry? Sheahan? Guentzel? I think Rutherford's gonna have to do magic to do an upgrade trade using solely Dmen and picks/prospects, so the question is what other roster assets do you move? How badly do you want it?
Me, I think the D can do it if the forwards play their part in the team system, and that a big name signing would get hung out to dry the same way if they don't. I wouldn't move Guentzel. But I would like to get a few more Hagelin/Rust type guys who have the energy and conscience to support our D most of the season.
edit: I'd add that getting Brickley, if he's all he's cracked up to be, would be a big move and I really hope it happens. Dunno if there's any Euro FAs who could play a big part quickly.