Not all the Sabres prospects will pan out. Obviously.
The point is, the Sabres have accumulated so many quality prospects from a massive number of high round picks that they don't all have to pan out.
Girgensons, Ristolainen, and Zadorov are already NHLers - that's already 3 for 3.
Sam Reinhart and whoever the Sabres pick first in this draft (almost certain at this point to be McDavid, Eichel, or Hanifin) are both slam-dunk 100% to be NHLers. That's 5.
Armia, Grigorenko, Pysyk, and McCabe are each more likely than not to make the NHL. But let's be conservative and say that two of those four bust and only two of them make it. That's 7.
After that you have a large group of decent prospects who all have a decent chance of breaking through for an NHL career. Lemieux, Baptiste, Fasching, Carrier, Compher, Larsson, Bailey, Karabacek, Ruhwedel, Ullmark, Makarov, Johansson - and that's just listing some of the better prospects. Let's say that 75% of that group of players busts out and never makes the NHL. That leaves three players from that group who make it. That's 10.
So even being fairly conservative and assuming that lots of guys will bust out, that's still a likely scenario of maybe 10 NHLers acquired from the draft in just a few years' drafts. That's a pretty rapid rebuild.