The game is in early access on Steam, but evidently you don't like it, so I doubt you'd care.
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The early access version on Steam has partial controller support. This means it's playable with a controller, but the menus dont work yet and you can't rebind any buttons for it yet. Those are coming later, but are not a priority.
While we will be looking at the possibility of console ports after the game is done, PS3 won't be happening since the current version of Unity doesn't support that and there isn't enough of an audience to warrant remaking the whole game in a different engine.
I chose to only release the early access version on Steam because keeping multiple storefronts up to date is a chore, so I picked Steam because it's more popular with mass audiences. GOG release is when the game is done and I'll probably also release it here then.
1. The full game costs money because I need to eat in order to stay alive and keep making games.
2. I don't think there's much reason to ever update the demo. The bugs in it are pretty minor and it's so far behind the latest full version that I'd basically have to redo the demo aspects of it again, which would be a timesink without much benefit. The demo at its current state still gives a pretty good baseline idea of whether you'll like the full game or not, so it does its job.
3. I only released the early access version on Steam because keeping multiple storefronts up to date is a chore, so I only wanted to keep updating one while the game is still in early access, and chose Steam since it's a far more popular platform for mass audiences. After the game is complete, it will also come out on GOG and I'll most likely release it here as well, but for the duration of Early Access, it will probably just remain on Steam.
Early Access just started on Steam, full release won't be for a long time. Itch.io is its own can of worms in terms of international taxes and such, so I have no idea when it'll come out here.