The answer is more kids and possibly higher taxes, rather than lower benefits. That expands GDP in the long term, so that in concert with more technical productivity, makes longer retirement - and indeed - younger retirement - more affordable.
Another piece of the long term sustainability puzzle is the development of self-sustaining environments with food production facilities. If people could grow much of their own food hydproponically, they could spend down their savings less quickly (and receive lower pensions where those still exist) - keeping active in retirement without taking a job away from younger workers. Indeed, as retirees age, they might even provide shelter and a job to younger workers so that they can assit them in growing their food while learning how to run a hydroponic house. This would be the perfect part time job for a young family or couple to take while still in training for a primary career.