Except for exceptional circumstances, only three results are considered valid for a chess game (and the corresponding rating change): win, draw or loss.
Your suggestion would come with a huge problem: what exactly counts as flagging? How big should your advantage have been for your opponent's victory to not count as a "real" victory? Who should evaluate that advantage? Would it be fair to take 20 seconds per move on a 3-minute game and still claim you dominated the match?
As other answers have pointed out, the flag is a part of the game. If you don't like losing on time, then you probably shouldn't be playing 1+0
You won't lose much rating for losing to a GM, whether by flagging or not, by the way.