Average is meaningless. A person's average has little bearing on their ability to play. If average went up when you quit a game it would be even more meaningless. Every sandbagger would quit every game to keep from being promoted to the next tier based on average.
I think to a certain extent the people who quit after a bad hole or two are of a similar mind to those who restart tournaments a hundred times trying to get a great score. The object is to get the desired outcome as quickly as possible. Any possible obstruction and they're gone. The sooner they quit the sooner they can try again with someone else. They're not necessarily quitting to protect average; they're quitting to win another game quickly.
I don't think many of them are even aware of the frustration they leave in their wake. The ones who do obviously don't care.
You already have the power to avoid those people who practice such behavior.
If you expect WGT to do something about it, the only way would be for the credits to be deposited at the beginning of the game. Leave and you forfeit your "investment." That would penalize quitters but unfortunately it also penalizes people who are disconnected through no fault of their own (game freezes, server disconnects' etc.) .
I just don't see any way that's going to happen.