As far as I can tell, the country constantly swings like a pendulum as whichever party is in power does a terrible job in their own special ways. If JFK had 8 years in office, it's far from a guarantee that RFK would have been as popular as he was.I think the dynamics of the 1980's would have been different if JFK/RFK had had a 16 year run. The country wouldn't have swung so far right and Reagan wouldn't have been elected. The white working class bloc that elected Reagan would have stayed more Democrat.
From a "Boston history" perspective, didn't the spigot of federal funding for urban renewal projects dry up after JFK was assassinated? If we had 16 years of those 2, how much more of Boston would be have demolished by the wrecking ball? We could have had another "anywhere, USA" city like Dallas or Houston, an unredeemable urban hellhole. So in the alt history, neither is assassinated, we have 16 years of them, DTX and all the surrounding area is demolished for a gigantic indoor mall, most of the South End gets the wrecking ball, North End gets the wrecking ball, and that highway through Roxbury is completed. Boston would no longer be seen as a destination, but in fairness, many other historic US cities would have lost much of their character as well. Montreal would be the preeminent historic city remaining in North America.