I thought of this possibility after watching GFIII for the millionth time this weekend.

After he makes his confession and then his bedside speech to Dontomassino, he turns over the reigns of the mob operation to Vincent. When he does it, he tells Vincent that once he is in he can never get out, and he further tells him that he had tried all his life to get out. As he leaves the room he tells Connie
he has given up his position as Don "Because I can't do it anymore." This is all after he swears he will "sin no more."

Before the opera starts, George Hamilton tells Michael that the new Pope who is an honest man and who is Michael's ally has approved the Immobiliare deal, making Michael one of the richest men on earth and completely legitimate. Michael realizes there will be a plot against the Pope by the bad guys and looks into whether or not he can foil it, but this is more lip service than anything else. Neri is already on the train to Rome per Vincent's instructions.

It is Vincent after all, not Michael, who plans and directs the murders of the Archbishop, Luchese and that Swiss Banker Fuck.

After Mary's death, Michael appears briefly as a broken old man alone in Sicily, but we can presume he lived part time in NY and Part time in Sicily, or at least went back to NY to sell his stuff. Obviously he had to arrange for people to run Imobiliare, and no doubt he sat on their board of directors, if not on several other legitimate boards. Presuably on Michael's death Anthony would not have to worry about getting paid for singing operas. With all his mob ties dead and gone except Neri who worked for Vincent, and was probably near retirement hiself, it is unlikely Michael made new connections with the mob so he could have a role whatever rackets Vincent got involved in. As a respectable businessman he would not have had anything to do with a re-formed commission.

So at the end of the day he accomplished (at a huge price) what he wanted. He was out of the mob, he was not invested in anything illegal, and he was a philanthropist an a prominent real estate player in the legitimate world. His primary relationshi was probably with Connie who would have no reason to be involved in the rackets, and I would imagine Kay probably didn't want to spend much time with him after his shenanigans resulted in Mary's murder.

This theory does not in any way take away from Turnbull's famous maxim "Crime doesn't pay," because as long as he was involved in crime it didnt pay. And the loss of a chid is a pain that never goes away. Still,once he got out of the crime business he did well financially and finaly became Legitimate."

I suppose you could speculate that he kept an eye on Vincent, but I think it is unlikely he would have had any interest in seeing how operations on "the street" were going any longer.