The real problem here is that Vito underestimated Solozzo's strength. He had the Tattaglias and Barzini, and probably the other families all ready to go into the drug trade. Sooner or later they would have begun to chisel into the Corleone terrritories all over New York...including Harlem, and Tessio and Clemenza's areas of interest. It would not be long until the lower ranking hoods under Corleone control would begin selling drugs themselves, and then Vito would really have had a mess on his hands..gaffe or no gaffe. The bottom line here is the Don was slippin'