Replacing tons of 1200psi D-type boilers and HP/LP turbines and their associated equipment (circ pumps, ejectors, steam turbine generators, forced draft blowers) with gas turbines is an attractive notion, but the cost of the refit doesn't fly. Too many ship systems suddenly need alternative motive force, and even basic hotel heat would still need to be re-thought.
Never mind the changes to the ship's CG and displacement.
The cabling changes required to upgrade early-70's sonar, radar and self-defense systems alone would be outrageous.
Newer platforms are also built to a completely different compartmentalization and environmental strategy to provide positive pressure boundaries in the face of chem/bio attacks, and the structures are better hardened against EMP.
The sound signatures and masking of newer-designed hulls far outshine the standards that we maintained in the early 90s.
Et cetera, et cetera, et cetera.
The Turks and the Greeks were thrilled to have our older ships, and I was just as happy to hand them the keys rather than see them cut up for scrap.