That still doesn't tell you if whoever pirated the game would have bought the game, which is the point OP was making. Sure, you can know if 60% of the players are using a pirated copy, but if that pirate copy didn't exist, what percentage of the pirates would have bought the game? That's the interesting question that's hard to get an answer for.
You can track diffs between releases of a franchise, if you have enough releases.
A release which gained 10M in sales which could not be pirated and scored 80/8 on metacritic can be compared to a release which sold 4 million but scored the same and has the same play time curves.
Obviously this is contrived, there are way more inputs, but you can see where I’m going.
Sorry, but even if you know that both releases should have sold roughly the same from past sales that doesn't mean it has to stay true forever. For example, the platform might've become obsolete, people got better things to play than rehash #9001, other sales channels became more popular, pirating to work around issues with intrusive DRM, dissatisfaction with past products, and many other possible reasons.
Maybe the full data is more convincing, but ultimately you can't really know whether someone playing your game would've bought it other than if they did. If, for example, each iteration of some franchise sells less with more pirating going on, it probably means your fans don't appreciate the way things developed and just want to see where their once beloved franchise is going. Assassin's Creed used to be one of the best in gaming, but for me it's not even worth pirating any more. Should've developed the current-year story like it was foreshadowed in AS2, but the whole idea of training Miles using the animus to defeat abstergo was jus dropped to milk the fans once a year. Doesn't surprise me when people stop buying in such a case.
