It's not that it must happen immediately. It's that, statistically speaking, it must happen eventually.
Detroit has already outperformed their draft slots with Raymond, Seider, and likely Edvinsson. They are seeing very encouraging development from Cossa, Augustine, ASP, and Danielson. But 1) there's no way that this will continue with all of their prospects, all the way to very good play in the NHL, and 2) even if it did, then you'll fill the roster and still have kids left over.
It's a numbers game at both ends. You start by hoarding picks and hope that enough of them hit to put some good pieces together. But you can't perpetually assume they'll all hit, so at some point you start to trade quantity for quality.
I just want Yzerman to keep his eyes open for opportunities. Don't force it, but take advantage of a good trade whenever you can. Because at least some of these "can't miss" kids will end up missing, and I'd rather package a few of them for a better player.