Interesting.

I always thought that the Don was being old-fashioned (and implausibly scrupulous) in not wanting to get involved in narcotics. However I also think that while perhaps the Don wasn't looking at the immediate next 10-20 years as Sonny/Tom were, he might have been looking REALLY, REALLY long term (30-50 years) and at the fate of the Families at a whole, and not just the Corleone Family.

While the narcotics business was very profitable, over time it also attracted an enormous amount of legal and political attention, caused criminal penalties for trafficking to go through the roof, helped build massive federal and international anti-drug organizations, weakened the control and discipline that top mob bosses had over lower level members and associates, reduced legal protections for those accused of trafficking, inevitably brought more drug users into the Families, and helped to make omerta something of a joke.

So I think the Don may have been thinking of some of this (for the greater good of all of the Families, not just his own) when he initially turned down Sollozzo's offer.

After all the Don was a statesman... \:\)

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.