So is the idealism, apparently. JFK's interest in civil rights was pretty brief and he didn't start showing much interest until what would turn out to be his last year in office. Assuming that the GOP would have put up Goldwater again, then yes, Kennedy would have won in a landslide, but I'm not sure that would have improved things that much. He basically would have replaced LBJ's term, so it's doubtful that the Supreme Court would have gotten much more liberal. And gay marriage was decades away back then, so it's unlikely that would have been accelerated.
I think it's unlikely that RFK would have had a better chance in '68 if JFK had lived. He was more popular after JFK's assassination than before, when he was considered by many to just be a crony. Agnew was the least of the nation's problems during the Nixon era. He was a loudmouth while more of the quiet guys were actually committing serious crimes.
There are some reasons to believe that the Kennedy administration would have serious difficulties if he hadn't been killed. At some point his sexual escapades could have turned into a real scandal. It could easily have been him screwing up the Vietnam fiasco instead, since he's the one who foolishly installed a corrupt autocrat there. And based on what is now publicly known about his many health problems, he might not have lived through a second term anyway.
At any rate, this is one of those situations where he died before he had a chance to really screw up, so he stays an idealist in people's minds. People like to wonder what Hendrix would have done if he hadn't died, but he might have ended up making crappy music instead. You never know.