No one working, everyone consuming, so we get either high inflation or high debt. If high inflation, old people need a strong welfare state or they die, so we get high debt. Young people need to pay for the debt of their elders, so high taxes. Productivity plummets because taxes are high and tax revenue decreases. Life gets worse for everyone.
One escape is that we have to drastically increase the retirement age. Getting rid of a welfare state wouldn't help, because they would still be consuming without being productive. Another solution is that we kill all old people. Another possibility is that innovation allows us to make investments that drastically increase productivity, but that's not guaranteed either.
As the problem gets worse, I think we will see this play out in formerly wealthy European countries and Asia, as their aging populations are further ahead than the US. Great opportunity for the US and third world countries to seize economic power from these places though.