Any quest for "legitimacy" on Michael's part was partly fueled by a healthy dose of self-delusion. By he end of II, he is not just a generic Mob Boss,but I would say that he has become THE Mob Boss.Of course,Michael wasn't subject to the kind of modern Media scrutiny that we see now, but I would think that his history would have never been too far below the surface. To divest himself of that baggage would have been a major feat indeed. Even by the time III rolls around and Michael has presumably no involvement in anything unsavory,there is still that stigma. The Archbishop mentions the Corleone family history,and the man at the shareholder's meeting protests Michael's involvement.Had he pulled off the Immobillare deal,no matter how rich and powerful he became,he would still be Michael Corleone.