Except that if you look at the dimensions of the blade and extrapolate via the direction of the edges of said blade one would come to the conclusion that the Guillotine's blade does not extend to the boundary of the track but merely fills the center section which would therefore mean that were the rope to gain slack the blade would indeed fall.
It looks to me like the blade is dull. Doesn't look sharp if you look VERY closely. That could just be the artists flaw though. Also, the rope that the blade is hanging on is completely straightened out. So it's actually just hanging in the spot its at in the picture. The only thing that scares me is that rope. It looks like its yanking on her dick, which means.... Goodness I get chills up my spine just thinking that. At least it isn't gross, but the thought of what could happen ruins all hentai for the time being. You know what I mean ^_^
it doesn't matter whether or not the blade has enough rope to fall completely, or whether or not it's sharp. her legs are tied to the track of the guillotine, and that rope will keep the blade from falling much further from where it is right now