My take, for what it is worth!

I believe Michael had little or no choice in taking over the business because it seems the Corleones needed to re-establish their glory, standing, reputation etc. that they are smart not weak and crumbling like everyone says, that's ain't the way they wanted it! especially Vito?

Pretty much Tiger by the tail for Michael

Among others,

1. Vito survived two attempts on his life
2. Sonny has been brutally massacred
3. Apollonia car bombed, meant for Michael
4. The Corleones were being trampled by the other families
5. Fredo Corleone was being slapped and straightened out in public by Moe Greene
6. Greene openly insulted Michael that the Corleones were being run out of New York and he'd been talking to Barzini
7. Barzini's people were chiselling into Tessio's territory and pretty soon there won't be one place in Brooklyn, Tessio can hang his hat!
8. Vito always playing the nice guy He even tells Carlo, Vito was happy for him [right hand man to Michael] knowing Carlo was going to be murdered
9. Vito's “And Fredo....well, Fredo was....well” not giving Fredo at least a face saving role in the family
10. Neither Vito nor Michael had any inkling what a time bomb Fredo was
11. Vito could have cut their losses and handed the business over to Clemenza and Tessio thus giving Michael a second chance for a non criminal life
12. Vito though never wanted the criminal life for Michael indeed left a murderous legacy
13. Vito and Michael's unrealistic expectation that their criminal business could be made legitimate let alone in five years
14. Neither Vito nor Michael realised how extremely difficult and elusive the legitimacy would be
15. Michael as a powerful man was responsible for other people – his capos etc.
16. I suppose Michel had to keep his caporegime going Otherwise he'd be just another casino owner and could be moved out like the Corleones did with Klingman