F-4G flew well into the F-15 era, and was damn good at its job. It was never replaced because the AF decided that LO would solve all problems, and we didn't need EA anymore. I think most would agree that was a bad decision. Do you want to start an A-A fight with the F-35 loadout? You're right, F-35 is more than LO, F-15X is also much more than just a new F-15C.
There are significant limitations to F-35 and putting all our eggs into that basket isn't perfect. Would I love to have 400 F-22s, yes. Would I love to have the F-35 able to solve all problems, yes. I disagree with your notion that there is no scenario in which I'd rather be in an F-15X than an F-35, but that is as far as I'm willing to go down that discussion.
When it comes to cost, the difference in purchase cost is negligible. This is all about sustainment. The F-35 is expensive as hell, the F-15X is cheap by comparison. Does anyone really think we're going to buy 1,763 F-35s? We need some cheaper aircraft with a lot of capability, F-15X is one way to work towards that future.