Perhaps he was. We need to keep in mind that Michael was not in the picture at that time and it was Sonny that was being groomed for the Donship. As one of the posters above correctly pointed out, Sonny did not have the ability to schmooze and keep those politicians in his pocket the way that Don Vito had. And Vito knew this. There are hints in the novel of the doubts that Vito had for Sonny suceeding him as the Don.

But I still think that Vito failed to see the bigger picture. Even his consigliere advised that the Drug trade would eventually make the other families more money which would make then even more powerful thatn the Corleones somewhere down the road.

Sonny was in it for the $$$. Tom had the foresight to see that they should get into it to retain their power in the future.