understand... listen, how GoG works? I mean, does that provide a DRM system? Do you need to download a desktop app (launcher) to play games?
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Is this a translation issue?
drm free is what it says on the tin, or else it would be a lie.
And of course GoG has a launcher. Even Itch has one. And the same as Itch, you are not required to use the launcher.
You do not need to look so far away for drm free paid games, just look at Itch top sellers. https://itch.io/games/top-sellers/store
Problem is, those publisher are unlikely to share their experiences here. So you will get mostly answers from the usual people that post regularly in the community message boards.
Maybe you find interviews or something from some publishers about the topic. Or something like that. Some very few games on Itch even do have drm. It's not forbidden, there is just no plug and play solution to it.
My opinion is basically this: people with a budget are willing to pay for games and services, even for "free" games, like the pay what you want games on Itch. And people without a budget simply cannot pay for games. This whole notion of drm is just bad as a concept in general for many reasons. I think the better concept is to upsell by having additional services, like cloud save games. Who cares if there is drm in it or not*.
* People that cannot play their games, because yet another ddos attack or another incompetent or disappearing developer, that's the people who care.