One-term president? Maybe he simply got found out. He was highly corrupt, but people see what they want to see. If the American public decide to have a fairy tale view of someone and his family, they will.
I agree JFK was no prize here on Earth-Prime, but we're talking about the idealized version of JFK from Earth-One, the president Superman took into his confidence like none before or after and made a personal friend and member of the "Superman family," appearing in Action, Superman, Supergirl and Bizarro stories (and probably more) and treated as a celebrity on par with Mickey Mantle, Marilyn Monroe and arguably the most revered celebrity in all of Silver Age DC, Jerry Lewis.
India's saying that for *that* guy to get killed would be as impossible as Jimmy Olsen or Perry White getting killed. More than that, it would represent a tear in fabric of Silver Age reality, a leg without which the table cannot stand.
Not that I agree, necessarily, but that seems to be where he's going. Certainly they stuck their necks out making JFK so prominent in the books, and I notice it's not a mistake they were quick to repeat. Curiously, after we suffered through the Crisis, ostensibly (at least in part) to remove some of the confusion generated by 5 decades of too-easily dated stories (the JSA meeting FDR and yet still looking young-ish in the 70s, etc), the first thing DC did right out of the gate was put Ronald Reagan in "Legends." Huh?!?!
Interestingly, after Reagan, we got a "President Forrest" (bigger version of "Bush"?) and then...well, after that I kind of dropped out. But anyway DC has a very spotty record when it comes to presidents, inventing some and sticking to "reality" for others. In JFK's case, it was a little of both.