This falls prey to the classic fallacy of "piracy" critics, that all instances of piracy are effectively lost sales.
In fact, there's little evidence this is so for most games. It makes sense that a game that can be pirated will be so by a higher number of people than those who pay for it, but this alone does not support the conclusion that the rates of purchase for a game that is extensively pirated are necessarily lower than if that wasn't the case.
In the case of The Witcher 2, after the patch that removed the onerous SecuROM DRM fully 4 million additional copies of the game have been purchased to date, compared to around 1 million copies total purchased during the few months after release while the DRM was still in place.
Obviously in no way does each instance of piracy equate to a lost sale, but even a moderate percentage of instances of piracy that actually lost a sale would be a disappointing total sum lost by the developer, considering the enormous overall rate of piracy.
Also, the 4 million figure is total sales of the first and second games on all released platforms, not solely PC nor following the removal of the DRM, unless you have a source that wildly disagrees with mine. It would certainly be an impressive vote of no confidence in DRM if 4 PC million sales had occurred subsequent to the removal of the DRM, however that appears not to be the case. http://gamespot.com/news/the-witcher-series-sales-hit-4-mill...
Considering how vastly pirated copies usually outnumber purchased copies, I hope you don't consider "a moderate percentage" to be something high, like 30% would be.
In fact, there's little evidence this is so for most games. It makes sense that a game that can be pirated will be so by a higher number of people than those who pay for it, but this alone does not support the conclusion that the rates of purchase for a game that is extensively pirated are necessarily lower than if that wasn't the case.
In the case of The Witcher 2, after the patch that removed the onerous SecuROM DRM fully 4 million additional copies of the game have been purchased to date, compared to around 1 million copies total purchased during the few months after release while the DRM was still in place.