You're forgetting that pre-Gen 6 Poison was a horrid offensive type and the only reason to even run a poison move was the secondary poison effect.
Not really, no. Well, it was a bad offensive type, but it wasn't just used for the poison chance. Most Pokemon that use those two Poison moves do so because they serve as a powerful STAB move and/or offer decent neutral coverage alongside its other moves. Venusaur runs Sludge Bomb for its power and the fact that, alongside Giga Drain and HP Fire, it offers neutral coverage against every OU Pokemon bar Heatran. Specifically, it helps with Dragons like Dragonite and Latias. For the few times Landorus-I used Sludge Wave when it was OU, it was because Sludge Wave was a great weapon against Celebi, Gyarados, and Rotom-W. Nidoking and Nidoqueen sometimes use the move since it is the most powerful STAB at their disposal. In fact, when Nidoqueen was RU, Sludge Wave was its best bet against Uxie. Roserade uses Sludge Bomb for the neutral coverage and STAB, giving it a solid attack for switch-ins such as Arcanine, Shaymin, and Victini, who might switch in expecting a Giga Drain. Haunter uses it, again, for the raw power factoring in STAB, as well as the fact that it doesn't have a lot of other viable options.
Regardless, whether Poison has good or bad coverage is irrelevant. The choice between Sludge Wave and Sludge Bomb has always been extra power vs higher poison chance. The only thing that has changed about that is the existence of Bulletproof.
Either way the instant Chesnaught's usage even gets remotely close to OU status you can be certain Sludge Wave will be Gengar's main option over Sludge Bomb. It's a lose/lose situation.
It's not a lose/lose at all. For all we know, Chesnaught (or any other Bulletproof user) may never see any notable usage in OU. If that happens, the choice will still be between more power or higher poison chance.