The game was financially successful, enough that we now have The Witcher 3 coming out also 100% DRM free. But the fact remains, a developer can do everything 'right' and gamers will still rob them blind.
Suppose they had DRM of the quality of a game like Diablo 3. Imagine all the extra sales they would have made (clearly not the full 5 million). Now imagine how good that money could have the people who work on The Witcher. You'd have more jobs, possibly a new IP that could be in the works. This is something that directly impacts working people and their families.
This is a logical fallacy that content producers fall into over and over. The idea that somehow you can convert those pirated copies into sales is nothing but a dream.
A sale you didn't make isn't a 'lost' sale. You're not entitled to some number of sales. You try to make money offering a product at a price that works for both parties.
Those dirty, filthy pirates are not your customers. They never will be. Stop trying to make them be. The number of pirates out there who would be pushed from 'paid $0' to 'paid $60' due to DRM is a tiny percentage of them.
Worse, if your DRM scheme works to convert some number of pirates into sales, then you've likely scared off a number of your legitimate customers due to onerous restrictions or annoyances. The net result is that you spent a bunch of money implementing and supporting DRM for no additional income.
When you step back and look at the situation with a rational perspective you realize, like the producers of Witcher did, that piracey didn't really hurt sales at all. They feel so strongly about that they're releasing ANOTHER sequel, also DRM free.