The whole concept of "stashed contributions" is wrong. Money is a fiction. It's really about resource allocation. In capitalism, whoever has money gets a say in how resources should be allocated, but that does not change the amount of resources available (if there is only 1 productive worker for every 2 retirees, it does not matter how much money those have "stashed"), nor does it make much sense to construct complex rule systems solely to reconcile the allocation of resources to retirees with the money fiction.

Sure, maybe "stashed contributions" is simplistic to the point of being wrong. It isn't that I'd like a better mattress to stuff paper under.

In my mind, money is an acceptable enough fiction that I can convince someone to allocate me a hamburger for lunch today. The fictional aspect of it is very handy in performing the time-travel of paying my future self part of my wages of today.

Times change and in the future a hamburger may require more or less money as it did today. My bet is that in the long run, that hamburger will be mostly the same or a functional equivalent will cost less because human ingenuity is an unlimited resource.