Ruined? You've got to be kidding.
The motto when the Eagle was being designed was "Not a pound for air to ground." This was consistent through the A model to the C model. But a funny thing happens when you design an aircraft with incredible performance; it can often succeed in secondary roles. This was true of the F-15, and it was also true of the F-14. This isn't something new, it harkens back to WW2 with the P-47.
Considering the success of the F-15 in all of it's models, I'd love to see what you consider is ruined about its design.