Other than Spielberg's giant crush on Le Beouf he's too smart to do this. He's also a busy guy, he's doing the Chicago (7?) movie + Tin Tin + his Lincoln biopic. In addition to it only taking 19 years to get the latest Indy movie made and George saying this was his last 'Macguffin', if they didn't use this idea he had no more left.
Also would Ford be willing to take a back seat? Would his ego prevent it or would he be just happy for the work?
Besides I'm more concerned with the quality of the 4th installment right now than a theoretical part 5. Maybe cautious is the right word.
Has this already been posted? I think its very appropriate to this topic. http://www.youtube.com/watch?v=CPzDjaA03ts
edit: We all know how George often talks out of his ass. This will happen around the time Star Wars Episode 12 is released.