Fuel savings does not offset development costs for the manufacturer. Sales from countries that want the fuel savings offset developments cost. Singapore is flying F110s in their F-15s and most foreign sales of F-16s use GE engines. Fuel efficiency if fighter engines is not the performance parameter that is is in passenger aircraft. GE, Pratt, and Rolls are coming out with new high-bypass turbofans for the A320NEO and 737-MAX that are a 15% improvement over the previous engine models. The advantages of the new engines are the increased fan diameter and tuning of the compressor and turbine aerodynamics. Only one of these would work in a fighter engine. Also the new re-engine efforts are combined with a lot of aerodynamic improvements to realize the cost savings over the life of the airframe (~30k hrs or so).
A re-engine effort for fighter aircraft would not be able to achieve the fuel efficiency improvement shown in the passenger aircraft because of the engine container limitations. The AF heavy aircraft (B-52, C-135, C-5) all have had space on the wing for the engines to grow in size.
Plus a 15% fuel efficiency gain would only gain the Viper something like 69 seconds of flight time. Not enough to make it worth while.