I think CDs could just as well be compared to retro games. They're basically an outdated, obsolete format (as much as I still love them myself), even if they were never replaced with a newer physical format for music. It's just that vinyl has that retro-cool factor than CDs still don't. So if vinyl is like collecting NES or SNES games, CDs are like collecting PS2 or PS3 games.
Anyway, if we're talking retro games that are pricey to begin with, I agree with jb143. I think the average person pirating them wouldn't buy even if piracy weren't an option. They'd simply move on to something else they could get for free or cheap. Now, when it comes to currently available official rereleases of retro games, where the games can be as cheap as a few bucks and actually profit the publisher and/or developer, I think piracy absolutely hurts sales. A lot of people think "If I've already downloaded and played the ROM, why would I bother paying for a download?" For example, Square Enix just announced they're localizing Romancing SaGa 3 for the first time ever, and they also just released the Mana collection for Switch in English, which is the first time Seiken Densetsu 3 has ever been officially localized. But how many people have already downloaded ROMs of Romancing SaGa 3 and Seiken Densetsu 3 and played through them with fan patches? How many of those people are going to buy the official English releases now that that option exists? How many will just be like "I already played the games in English and don't care to do so again, so why bother buying them?"