It could be complacency but it could also be that the Don's opposition to drugs "so long as your business doesn't interfere with mine" would have operated as a veto (pardon the pun) in years past, but now, with the other families especially Barzini and Tattaglia wanting in there was a feeling they could out muscle the Corleones and have their way. Remember that after the Don was shot, the Corleones really lost the ensuing war. The Don was the one who ultimately gave in, and it was not only because Santino died, but it was also because the price of more blood was too high. The Don admits as much when he realizes it was Barzini all along, because he refers to the fact that they "outfought Santino." Later Moe Green mistakenly confirms this "conventional wisdom" when he thinks he can push Michael around by telling him "the Corleone family don't have that kind of muscle any more," and "You're being run out of New York by the other families," and "I talk to Barzini, I can make the deal with him and still keep my hotel."