I'm mostly valuing Cousins, not Noel. Cousins is one of the premier players in the league, and a far better player than McCollum, if he's straight. The fact that he's not always straight is why he'd be available at all. If Cousins had interest in being in Portland, I think the Blazers win that total exchange pretty easily which is why I've said, in a previous post, that Portland would need to be able to talk to Cousins first.
And while losing the team's only real starting-caliber small forward and shooting guard definitely unbalances the roster in terms of positions, it re-balances the roster in terms of being manageable to build a real defense in addition to offense. The Blazers are almost certainly never building a strong defense around two terrible guards--that's simply punting too much perimeter defense in a league full of attacking guards. Having Lillard and Cousins as the foundational pieces brings into the mix a center who's capable of being a good defender in addition to being a great offensive player. Obviously the team would need to make further trades to divest themselves of bigs and acquire new wings, but that's a far easier task than putting into place a star tandem that you can build a champion around. If the star tandem is Lillard and McCollum, I personally don't think Portland ever gets past the stage of "fun, awesome offense that loses too many shoot-outs to challenge for a title."
I think people in general undervalue defense on the individual level. If a player is a great scorer, that player's defense becomes an afterthought, even if that lack of defense erodes a lot of value.