This is a fight to maximize the value of both fighters, but particularly Silva since he's nearing the end.
PEDs are not an issue since 1) Silva just got popped on them, and is now under immense scrutiny, and more importantly, 2) UFC's new stringent drug-testing measures will go into effect by then. If anything, Silva coming off of PEDs (allegedly) will hinder his performance, ala Belfort against Weidman, and will be an advantage for St-Pierre.
Not only that, the fact that Silva was busted for PEDs, and St-Pierre specifically left due to the inadequate drug-testing measures, paints a clear antagonist and protagonist in this dream fight.
This is a fight UFC should book for business reasons, and it would most certainly do excellent numbers—which is something UFC could really use after a few years of low buyrates.