Where do all the tough old birds go?
They go into soup.  Or pot pies.  Or any of the other myriad industrially produced canned or frozen foods which require chicken stock or chicken flavoring and sometimes canned chicken meat.

The millions of cans of Campbell's have to come from somewhere :-)

For smaller home chicken farmers, they probably end up in the stock pot.  If you know people who keep backyard flocks, they may be willing to sell you their culls for a very reasonable price.