I doubt you've got a good data stream about sell-by dates, tho - the old and new have the same UPC generally. Maybe OCR of sell by stickers? And this doesn't help things that don't have dates, like produce.
It's been about 12+ years since I've regularly dumpstered food, but my experience is that more food was being thrown out before (but maybe not much before) the sell-by date than after.
I think the issue is that of given the choice between something with a sell-by date a few days in the future or 10-15 days in the future at the same cost, nearly everyone is going to take the food with the better date. Which means the arrival of a new batch of inventory makes the older inventory barely salable.
Technical solutions could help here: it is taxing on humans and most POS systems to have to adjust the price of older inventory, but if that could be done automatically (or the labor pushed onto the customer to identify the condition in exchange for a discount) people looking for deals might help reduce this type of waste.