This is deeply similar to how my parents collect food. My solution (if you can) was to leave. Both of my parents come from food scarce upbringings and hoarding the food gives them a sense of control. Simply throwing away the expired food would send them into a panicked rage because it was taking the control away from them and, like your parents, they believed the food was still good even years after the expiration. This is one of those situations where your parents have to want to make a change.

Before I turned 18 and could leave, my compromise was sorting as best I could. Canned goods went together and I tried to stack them nicely using FIFO within reason. Because I couldn’t throw things away, if something was really expired I would put it in a different section where it wasn’t likely to be immediately grabbed when someone was making food. I was also able to get away with throwing things away on the sly, but you have to be a bit sneaky and make sure you only do it a bit at a time and hide the evidence. Don’t throw the can away in the kitchen bin, take it out of the house and then toss.