The F-15 is indeed a proven design, but against modern integrated air defense systems it is going to have a really, really hard time without extensive support f

The F-15 is indeed a proven design, but against modern integrated air defense systems it is going to have a really, really hard time without extensive support from other platforms (EA-18G for protective jamming as just one example). And even then the crew of an F-15 may find themselves in a situation they can't survive. Even if F-35 isn't the answer, 4th generation aircraft have serious survivability issues.