Obviously, Tom would be useful, not just for political purposes but because he knew where Vito's money was. I think that his outlook is grim, though. He's with Luca in the category of people whose lives were saved by Vito. He could shift his loyalty to Michael because that served Vito's wishes, but he wouldn't be able to transfer loyalty to Vito's enemies. He could eventually go Sicilian and try to shoot Sol or Barzini. He'd also be a huge prize for the government to turn, and Barzini would realize that. About the best scenario for him would be that they decide enough blood had been shed, get the information they needed, and then let him go to make a quiet living as a lawyer in some corner of New York State. I think that's a longshot, though.