Those that keep the stamps and never use them allow the USPS to turn a nice profit. They love philatelists but the hobby has suffered because of national post offices churning out tons of issues over the years simply to milk the stamp collecting market. I stopped buying new issues years ago because of this. Is is similar with many other collectibles. There was a thread some time ago listing collectibles that will never gain in value because of overproduction.
I still have my collection that went from the 1900s to 1970s, there was always be a market for those stamps.
I was in a shop that sold guns, coins, jewlery, stamps, etc. A lady came in with a big box of “collectible issue” stamps and wanted them appraised. The owner said he would pay face value for sheets of stamps and didn’t want smaller lots or individual stamps. He said nobody collected most of the ordinary collector issues and only wanted rare, old stamps. He used the sheets of stamps he bought for ordinary postage and showed me some of his outgoing mail. The packages were covered with stamps.