There does not exist a single peer-reviewed study that shows a correlation between increased sales and DRM systems.
World of Goo was one of the first indy games to reach large reception of users. Sadly, it takes time before people are willing to spend money on new concepts. Same effect can be seen in cult movies. Most cult movies do not earn a bunch of money. They barely even get copied at first. Blade Runner did not have an issue with piracy, and they still got crap in revenues. I would not be surprised if 90% of the people who have seen Blade Runner did not do so at a cinema. I would guess most people has seen it on a copied tape, copied by someone who thought "Ooo, what is this? this is nice. Let me share that with a friend".
Had World of Goo been released today, they would had much better sale numbers together with current piracy numbers. World of Goo of the past has shown that indy games a worth the risk of buying a copy and testing it out (if one would ignore the paradox).
It should also be mentioned that creator of Psychonauts got more (or close to it) money from kickstarter, than from initial sales. Ie, people were more willing to bet money on something for little or no return, than they were willing to bet money on the original game itself before it was known to be good. Do anyone actually think that DRM would have made the original Psychonauts a massive revenue source?