You reach a point where if nearly everyone is blackmail-able for the same thing, that it stops becoming effective. If everyone is cheating on their spouse, then who is anyone to judge?
Also, there is the possibility of creating false data that is worse than the average blackmail material. For example, say you are a political dissident starting a new party that appears to be an actual threat to the incumbents. Oops, we leaked the data showing you had an uncanny knack for finding and repeatedly viewing the most illegal of material before our moderators manage to take it down.
No matter how well you can defend against such a claim, your political future is now over.
And with porn it's so multifaceted. Everybody breaches some taboo or other, but they're all different taboos. Not to mention that every social taboo immediately gets sexualized.
If the conspiracy theory was true though, they would have never let Tumblr go down. Tumblr was the mecca of weird, fringe, taboo porn. And they could even take down the people who made it.
That's not true, because it requires a further massive step (that is unlikely to occur): all people must be similarly revealed to be doing the thing in question.
You'd have to know that everyone is cheating on their spouse; everyone would have to have max knowledge / transparency. Until then, the ones that get revealed will get judged by those not yet revealed. In my observation, people tend to be very comfortable with being a hypocrite, right up to the point where they're burned by it.
And further, if there's even a small detail that is different, the crowd will use that detail as the point of differentiation to say that they're not as bad as some other person.
If you have 100 people all surfing the same porn, and only one of them gets publicly revealed for the behavior, most of the remaining 99 people - assuming individual separation - will pretend they're not doing the same thing. They'll ostracize or harshly judge the person that got revealed publicly, pretend they're not doing the same thing, and go on doing the same thing in private. This behavior seems to repeat essentially non-stop among people socially; you see it in the news, with politicians or celebrities, with scandals, in various group behavior, among friends & family, et al.