Bear in mind the reasons that the MiG-21 was successful against the US over North Vietnam - it was over North Vietnam.
Sure, the VPAF was on the defensive and heavily supported by a SAM and AAA defensive network. But then the US had vast had vast numerical and technological edges that the VPAF had to work carefully around if it wanted to avoid stuff like Bolo being the rule rather then the exception in '67. So it picked its fights carefully, focusing on laden strike aircraft more than fighters. But this strategy was hardly unbeatable: had the US leveraged it's resources better, the air war in '67 as a whole could have gone a lot better for it. In fact, that is what happened when the US started leveraging it's resources better. After the fiasco that was the overall air war in Rolling Thunder, the Americans went back to basics and gave their pilots a level of intensive air-air combat training using resources the North Vietnamese could not have hoped to match. As a result, when the USAF came back for another go in the early-70s with the Linebacker raids, they generally kicked the VPAF's ass up and down the street without needing to rely on the sort of set-piece operations staffed by select elite pilots that Operation Bolo represented... and which largely left the USAF in the lurch when Old's and his cadre of experienced pilots departed in the summer of '67 and were replaced by pilots poorly trained in the vagaries of ACM.
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