Sure, a straight GDP measure would have the methodological problem you point out

One reply to that is to use different methodologies

The canonical reply is probably something on the order of “people can’t sustainably engage in practices where they exchange $1 of value for $0.50 of value, as they still need to exchange labor for food and other real goods.”

You normally see the latter play out through credit crises, where the benefits of some activity fail to materialize and therefore loans can’t be repaid, and a lot of pain follows.

Under that view instantaneous measures of GDP don’t mean much, but measurements averaged over longer periods, credit crises, etc. take on some meaning.