Yes, it's that simple: he could have walked away.
He was blessed by the Pope with a Knighthood; he was the benefactor of Sicily; and maybe he folded his hand with the Commission in AC.
Why did he need Immobiliare? And all its risks and conflicts? And its threats--to his life and to those around him? And, therefore, why did he need Vincent and his thuggery?
Ambition. He wanted to be the next Rockefeller. The saying "There is no great fortune without a great crime" carries much truth, and Michael wanted to clean the Corleone name and be legitimate ...and rich.
Again it was Connie who approached VIncent and said if anything happened to Michael he had to take over. In that scene Vincent said he had already begun making plans. At the end Michael did walk away and Vincent took over, but yes, he walked away too late.
