You need to have three scoring lines. Your theory precludes that from happening. We can't afford to be a top heavy team anymore, that formula doesn't work in the playoffs. Better to have three capable scoring lines. We can still fortify the top six in other ways. Sutter does not have to be the player traded to accomplish this.
edit: I also don't agree that our window is another five or six years, that's a very optimistic timeline. three more years, maybe. After that Sid and Geno our likely to drop off, at least a bit.
You also need to have enough top 6 talent to give the 3rd line better match up's, otherwise having 3 solid scoring lines doesn't mean much.
I'm not sure the drop off when Crosby/Malkin are in their early/mid 30s will be enough to close their window.