Ok, so let's look at a recent release on PC: Dragon's Dogma.
Now, this is a port of a 3 year old game with a cult following that for some reason didn't get a japanese release. It's also being sold for $30 after all this time. When it initially released in japan it got ~393k sales, when it intially released outside in the U.S. it got like 90k-ish sales in launch period.
Now, for the PC release, 3 years later mind you, the week before the game is released one of the reviewers releases their personal DRM free review copy and it's spread around on pirate sites. Despite being pirated before it even released and being an old game that didn't review particularly well, either, the game still manages to sell over 200k in the launch period, half as well as the initial launch in japan, and more than double the U.S. release.
But sure, pirates hurt sales to the point where devs shouldn't consider PC releases. Let's go with that.