Considering that we still have superiority over anything else with such birds as the F-15, why not keep the F-22 program around and even advance that but also keep producing F-15s and improving those as well. I'd hate to see advanced aircraft not be developed but I'd also hate to see us with too few birds due to high costs.
It isn't a kill ratio that makes the aircraft so great once a seriously superior kill ratio has been established, it is getting those birds to every place they need to be around the globe. Too high a cost means fewer birds.
The new SU's are every bit as good as the F-15, which is after all a late 1960s design. Even the F-16 and F-18 are early '70s designs. I remember the flyoff between the YF-16 and YF-17 (predecessor to the F-18). The threat evolves, your systems must do so as well.
Wasn't the F-15 C/D line closed a long while ago???F-15Es are still being built for export orders,none for the USAF.