FedEx and others are still operating 1970-vintage DC-10-10s (albeit converted to MD10 freighters), and GE is glad to keep supporting their CF6-6 engines at a fair profit. Likewise the LM2500 for industrial & marine apps, and the TF39 engines on USAF C-5's, although the latter aircraft are finally being upgraded to CF6-80C engines.
So P&W JT3D fans on a DC-8 are a very close cousin to TF33 engines in the B-52H, and the USAF expects to keep them flying another few decades. It is said that the mother of the last B-52 pilot has not yet been born.