We all may realize that we will die some day, but few of us can really visualize a world without us in it. Vito was no exception. He wanted Michael in a legitimize business or position, but only as Vito defined legitimate. Vito wanted Sonny as his successor (simply, primogeniture). Thus, Vito would have best of both worlds.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."