Two points:

A telling line left out of the movie but in the saga is in the scene when Michael first comes home and tells Vito that his giving in on the drug issue could be construed as a sign of weakness. Vito corrects him and he says is IS a sign of weakness, so however the family was being run, they had a sick Vito who lacked the necessary stamina, and Tom and the Capo Regimes trying to hold it together. SO however things were run after Sonny's death the family was certainly not gaining strength.

As for the point about what would have hapened to Michael if Sonny lived, that's a tough one. I think Michael with his
"I'm with you Pop" and his murder of Sollozzo and McCluskey "crossed the Rubicon" and could never go back to become a lawyer or a Senator. Not shown in the movie is how someone else took the rpa for what Michael did, but I don't think anyone really believed it as witnessed by the questioning of Michael years later in the Senate hearing. He was way too mobbed up to hold public office. Besides, Hagen was already there as a lawyer, so what role Michael would play with SOnny alive is tricky. My best guess is that he would come into the family business and use his guile, cunning and intelligence to maneuver Sonny to do his bidding without Sonny realizing it.
Additionally Sonny being the loose cannon that he was there was a good chance he'd have done something else to get himself killed anyway.
Also, had he lived and Michael returned even Vito could have put Michael in charge of the family business and move Sonny into the background as consigliere or co-consigliere. He could say his affair with Lucy Mancini and the birth of Vincent Mancini was an infamnia for which he had to be punished.