Working longer? That would violate the contract they have with government and society to be able to retire at a certain age. If the government fails on it's promises the social contract fails and it's major disruption time. Let them raise the retirement age, so what?
If not, then the people will have a revolution, redistribute the wealth, learn from the mistakes of the past, and then start over with a system that will work better.
Civilzation progresses in fits and starts.
Either is preferable to the sisyphean solution of immigration.
Actually what will happen is a patriotic duty for assisted suicide so that you aren't a burden. Also if you don't want to then the state will help you. They'll put you on an ice flow so that you can a nice all expense paid trip in your dotage.