Indeed. The Confederacy loses after his death (just like they lost OTL, only with the timing changed), and the Lost Causer temptation will be to blame it not on Lee, but on Lee's death. The meme will be "had Lee not died at Gettysburg, surely he would have snatched victory from the jaws of defeat, and saved the Confederacy."
The Lost Causers will probably claim that only the death of Lee and Jackson within a few months of each other allowed the "Damn Yankees" to win, and both will have even more of a cult of martyrdom around them.
As for successors, remember that Johnston has just failed to relieve Vicksburg, so Davis will be even less willing to entrust him with command than OTL. I agree that Longstreet and Bragg are the two most likely options.