The supposition that few/not many people in the U.S. are reusing postage is a fantasy, I believe. I would say that roughly 20-30% of my
ebay purchases are franked with reused stamps. How do I know? If I try to remove them, the glue is "not right". Stamps applied with white glue (PVA) will not separate from envelope paper. More simply, many of the stamps are "shopworn"; they appear used on casual viewing. Some even have traces of cancellations.
I haven't seen any figures on losses by reuse to the USPS, but the figure has to be tremendous.