Take civil rights legislation in the 60s; a lot of what LBJ accomplished was credited to the national feeling after the Kennedy assassination. Would JFK have been able to accomplish all that if he had served eight years with public sentiment toward civil rights being divided?

I think the same thing about the Apollo program. Kennedy gave that speech and then died, and now not only did we have to beat the Russians we had to do it for a beloved slain president.

There's also an episode of Red Dwarf where they go back in time and convince Kennedy to be the second shooter on the grassy knoll, because his survival would have started WWIII.