When looking at individual aircraft choices (e.g., the decrepit F-15C fleet), these are (mostly) very reasonable, palatable, and defensible decisions.
The F-15C has never been shot down in combat. Unprecedented in the history of combat aircraft. But ok, “decrepit”.
Stealth is the only technology that provides any safety in a modern air combat environment. The F-15 is a wonderful aircraft, but it doesn’t have any stealth capability. That’s why the F-22 and F-35 are far superior to any Gen 4 fighter, including the Mig 29 and SU-27.
Only the US, Russia, and China have Gen 5 fighters. Everything else is just a target now.