One thing I can agree with Kennedy on is civil rights. Johnson has never shown a willingness to do so. He needs to go.
If he does, who will Kennedy replace him with?
I find it unlikely that Kennedy will replace him. He needs Johnson to hold the south, and no Democrat can win without the south. Either that, or he has to with someone like Russell or Thurmond, and we can all agree they would be worse.
He could pick Connally, Shivers, or an old timer like Coke Stevenson. Johnson isn't popular, but he controls the machines and many politicians. It would take a United front between Sanford, Smathers, Symington, Shriver, as well as Robert Kennedy, Pat Brown, and the other Northern and Western Democratic leaders. For now, the combined support of Sanford, Smathers, and Symington is almost assured - they're staunch Kennedy allies.
As for Johnson on civil rights, I'm sure he would pass it, if only because he's the biggest opportunist in politics, which says a lot.