A good rule of thumb is to ask "are you paying mostly for human labor or for machine labor"? The former is likely to be more expensive now than it was in the past and the latter is likely to be less expensive, all relative to general inflation prices.
A hot dog / hamburger at a diner is mostly human labor, so you'd expect it to be cheaper in the past.
Labor is typically around 30% of the final cost of prepared food in a restaurant.
Remaining 70% is 30% food costs (which has dropped drastically since the 50s), then 20-30% operations. Profit is whatever is left.
So a diner burger is not mostly labor but I honestly have no idea what these costs were 70 years ago. I'd love to know, seems like something is missing.
Likely everything in the chain going up 1-10%.
Food cost hasn't dropped because you can't even get the food they used to have. You have something that costs less now, but is worth even less than what it costs. And now that Sysco has completed it's eradication of all variety and competition, it doesn't even cost less any more.
Much of the 30% food costs is labour further up the chain, and much of the 20-30% operations is also labour.