The transition from a larger population to a smaller one means the less workers will have to temporary fund the social programs of a large number of retired people. The retired people will also be the largest voting block and will dominate policy-making to force funds into the social programs that they worked for. Depending on the shock, this can cause total breakdown of the system. That's why everybody is racing now "to sustain the expansion at any cost" and to maintain unemployment low for as long as they can. Once people start retiring from this cycle in particular, nobody knows what will happen.