Granted. You'll have a nice sandwich, but not now. You'll never know when the good sandwich will come. Is it the next one? In a year? And when you get a good sandwich, will it be your last one.
Enjoy the anxiety.
alternate ending: You don't know if you've already had it. Maybe that decent sandwich was the good one, and no other sandwich will be as good. Until you get that better one, after which maybe THAT was the good one
You get invited to a restaurant that is only open once a decade. You get one sandwich. You are given the “good” sandwich. You eat it and look in the kitchen on the way out. You see a severed hand in the meat slicer.