Is it possible without a Kennedy run in '68 that McCarthy is able to pick up some commanding delegate leads in California,Nebraska and maybe Indiana? Eugene also had the lead in Popular vote in the primaries so could Hubert Humphrey actually step on his own feet and mess up big time to wear the old Party Bosses conside endorsing McCarthy instead.?
McCarthy was unacceptable to the party leadership (and quite unsuitable to be President, in all honesty). If they moved against Humphrey they would draft somebody instead of supporting McCarthy.
No RFK means McCarthy (assuming he doesn't implode) wins California and New York but not Indiana, and does a little better overall. It's not enough.
Now if you want to talk party leaders, IOTL Mayor Daley pushed LBJ and the others pushed Teddy Kennedy which Daley agreed with once realizing no one would back his LBJ push. EMK turned them down, and Humphrey won. If EMK had accepted it's probable he would have received the nomination.
In the ATL if Humphrey comes into the convention without the magic number (around 1300, if I recall correctly) because McCarthy did well himself and in keeping unpledged delegates unpledged than the party leaders will confer.
It's almost certain that Daley will push LBJ, but back down and accept his close friend RFK if the others push him. Would they? They went for EMK IOTL faced with McCarthy & Humphrey, I see no reason they'd discount RFK.
Would RFK agree to a draft? I'd wager probably not, but at least in these circumstances McCarthy would be willing to throw his delegates to RFK if it looked clear that RFK could win and McCarthy could not (IOTL McCarthy grew to hate RFK with insane passion).
So, amusingly, by not running RFK could become President

. (Odds are Humphrey wins and than loses to Nixon though.)
The two question that follow would be "would Bobby run in 1972" and "if he runs in 72, can he win".
If he thought he could win. If he runs, he thinks he can win so… yes.