I am surprised that people still say, "what if everyone did that?" It's a dumbed-down version of Kant's categorical imperative, and gains a bit of validity through that association even as it loses its force with the lack of nuance. Because the most reasonable answer usually is, "well, not everyone does (or will)."
Leaving aside the tragedy of the commons for the moment, I want to look at the one place where it's just a dumb argument: pacifism. As in, "I know Quakers don't fight, but what if everyone did that?"
Um, there would be an end to all wars?