Right. Cos PC sales would only be cannibalising console sales, it's not like it's an whole other market of customers to market to.
Let's take a recent example of a big name game that was also released on PC, Monster Hunter World. I believe Monster Hunter World is still averaging over 100k players a day on Steam. It has not been hacked to my knowledge, yet it's been out on PC for some time. It sold, according to Capcom, extremely well.
https://www.pcgamer.com/monster-hunter-world-pc-sales-have-exceeded-expectations/
It released some time after the console version, but had it released simultaneously none of your dire predictions would have come true. It hasn't been hacked, torrented, or illegally distributed, so it couldn't have lost them sales.
Piracy really isn't as big a deal as some would have you believe. The Witcher 3 was released on PC without any piracy protection at all, because CDPR doesn't believe in it. It was resoundingly successful. Why? Because most people are happy to pay for a product that they like. Those that can't afford it were never your target market anyway.
I don't think you can make the blanket statement that Rockstar would absolutely lose sales with a straight face. They might, they might not (although with any knowledge of the current PC gaming scene it seems unlikely). It depends on how strong the PC sales are, whether they choose to protect the game and whether that protection is quickly cracked. And honestly, probably a bit on the reputation of the company. I think someone CDPR gets a lot of people to buy because people want to support their anti-DRM/pro-consumer stance. I think someone like 2k Games and 2K18 get a lot of people refusing to buy simply on principle because of their shady cash grabs.
I think someone like Rockstar would probably be pretty firmly to the CDPR end of the scale; I hear a lot about how abusive the microtransactions are in GTA Online, but honestly GTA V was and is an incredible bargain of a game without any online at all. I think people want to give Rockstar money for the great games they make, which is why there's such anticipation for RDR2.
I think with Steam you have a whole extra market of gamers who aren't necessarily console owners. I think, based on my observation of the success of other multi-platform games, that it's pretty universally a net win for a company to release on PC unless they're a really tiny company and can't afford the dev costs.
That's why the vast majority of AAA games these days also release on PC. They wouldn't if you were correct, and launching on PC cost them sales. The only real reason for AAA games not to release on PC is console exclusivity, and that's just trading money from PC sales for money from the console manufacturer.
So yes, unless Rockstar somehow have an exclusivity deal with both consoles that bars them from releasing on PC, I'm sticking with it being an irrational choice. They're literally avoiding money that they could otherwise make. That's irrational.