I believe had JFK completed a second term Robert would have followed, not the Texan.
In the spring of 1968, Democrats were fed up with Lyndon Johnson who read the tea leaves & bailed after Eugene McCarthy beat him in the NH primary. He died only four plus years later.
The nomination & the Presidency were RFK’s for the taking until fate intervened. Hubert Humphrey never stood a chance.
It is not clear that Kennedy would have been re-elected.
He was not that popular before he was killed.