I read an article a few years ago that stated that there are enough spare engines to see the B52 through to about 2045 and a re-engine program would not be cost effective.
So 80+ year old bombers.
Also there are many engine types in service that have been out of production for years. You just keep overhauling them. The spare parts will still be made as long as there is a demand for them.
Mind you some of these engines could be like "Trigger's broom". Apart from 3 new heads and two new handles...........................