The novel states that Sonny was anxious for an operation of his own, "to get out from under his father's thumb." So I infer that Vito might have worried that Sonny being involved with the drug trade--and running it after Vito's death--the entire Corleone enterprise would be in jeopardy.

Tom told Sonny (in the film) that if Vito died, "we lose half our political strength" (or something to that effect). Why? Possibly because the police, politicians and judges who trusted Vito's judgment and wisdom wouldn't have trusted Sonny, whose reputation as a hothead surely would have reached them.