And when Luck sucks what then?
I'm not sure what you're even hinting at here. You can say this about any draft pick that you take at pretty much any position. Could have said "What if Chad Henne sucks?" a few years ago, and guess what, yep he sucks. No player is ever 100% guaranteed to pan out, but Luck is the closest thing to it. That and you have a much higher % of hitting on a top QB than you do scraping the bottom of the barrel.
If Luck turns out to be a bust, which btw has about the same odds of happening as the Dolphins making the playoffs this year, then you reload in a few years and draft another QB. You cannot win in the NFL in this era without a star QB.