Well, how far goes loyalty in general? I mean, if there has ever been a loyal person in history or fiction, it was Tom Hagen. But there's always a limit. And I think his loyalty was tested more often than that of Jesus.
Many times Michael has betrayed the memory of his father. So if Tom happened to distance himself from Michael it could be one way of keeping his loyalty to Vito.
Good points. Michael's exclusion of Tom from Family business, his emotional manipulation and humiliation of Tom, forcing Tom to carry out the "hit" on Frankie, the fact that Michael had either Rocco or Neri spying on Tom's personal life, and just his overall coldness and disdain for someone who was not only an older "brother" but a pretty important power broker all had to grate on Tom tremendously.
I could see Tom starting to give less of himself to Michael. Tom could just give the facts and the absolute minimum asked of him, offering no advice or analysis (since it would be ignored anyway), and showing up less and less at family get togethers-brothers do that all the time in real life.
Maybe ultimately he would have left but I don't see it.
