That's like imagining a world in which everyone became nurses and we had no other professions, or where everyone decided not to have children and the population crashed. We shouldn't discourage people from becoming nurses or not having children just because it would be bad if everyone did. If economies really started crashing because everyone gave away lots of money (or we had too many nurses or not enough babies), people would adjust their behavior. Organizations like Giving What We Can and The Life You Can Save advocate those of us in the developed world giving between 1% and 10% of our income. That would easily end the worst of world poverty, and (I believe) would not destroy economies. Obviously, people in the developed world are not leaping to give away 50% of their income. Until they start, I'll continue trying to make up for them.

It is only necessary to give up to the point where the problem is fixed. If everyone giving all their surplus would be non-optimal, there is some amount less than "all" for each person to give that would be optimal. However, most people are not giving anywhere near this amount, so in the here and now the world will be better if you give all your surplus. If the world were different, the principles that led to this advice would lead to different advice that would be optimal in that situation.