McNamara and his notion of commonality;
F-110/F-4 - genius idea
F-111 - disaster
I'm sure the US taxpayer would have greatly appreciated wasting millions of dollars turning the 'one-oh-wonder' into the 'one-oh-whatever' when McDonnell had, of course, already done the work in creating the F-4 for the Navy.
Historical Note - In no way did McNamara 'inflict' the F-4 on the USAF. In 1961, almost as soon as he got his legs under his desk, he did order a series of direct competitive evaluations against USAF aircraft and the results showed that the F-4B was better than anything the USAF had on every count - mission, performance, maintenance and serviceability and also highlighted that the basic design was, potentially, the world's best tactical reconnaissance aircraft. That's why it was ordered for the USAF. For the avoidance of any doubt, I am in no way praising McNamara but I am praising the qualities of the F-4 in the era in which it was born.