Because of delays in the F-35 program, there will be a shortfall in fighter strength for the USAF over the next few decades, so they are looking at service-life extensions for existing F-15s and F-16s, and they have flatly ruled out any purchase of new build legacy fighters.

The idea here is that if they buy them, Congress might be tempted to cancel their white elephant.

BTW, speaking of white elephants, it not looks like the USAF will have to have its other white elephant, the F-22 will have to be escorted by F-15Cs upgraded with AESA radar, since the F-15’s larger antenna size allows for greater jamming capability. Basically, bigger antenna means more AESA modules which means more power and more range, and the AESA functions as a jammer.

Additionally, the F-15, “have the persistence [because of larger fuel loads] to stay there while the [stealthy fighters] are conducting their LO attack”. Note here that the F-15C is about ⅓ smaller than the F-22A, but apparently out-ranges, or at least out-lasts, it.