Lousy shooting is no excuse to go up in gauge, but I agree with you, most people take shots that are too long, don't swing the gun properly, and don't look for the beak of the bird when doing so.
However , all things being equal, a 20 is fine over decoys as long as the dammed shooter knows how to shoot.
I don't hunt with guys more than once that brag about their tens and cannot match my 20 when we get into the blind, but if they ask I sure will explain how one should be shooting at birds and we will go out again.
I hear the same thing about semis and my hammer doubles - apparently a BP gun loaded with black and non tox is no good for geese.
Well, it isn't , I guess, if you are trying to shoot birds with it a 60 yards and
aiming it instead of concentrating on the head of the bird and swing the danged gun!
I may not kill as many geese as some guys do, but I'd lay odds that I shoot far less in the breast and wound less birds than a lot of guys simply because i show a bit of restraint on my shots at times, and practise constantly in the off season with the same guns as I hunt with ......
Cat