Leroux: Yes, I agree. But as a dev you probably can't win this either way. If you don't bring your game on GOG, you're an ass. If you give in and do it, but you regret it later on, you're an ass, too.
Then again, if GOG had rejected Supraland (e.g. because GOG wasn't sure the developer was committed to supporting the GOG version), everyone and the developer too would have also shouted bloody murder.
To me this sounds like a good approach for developers considering releasing their game on GOG:
1. Only release your game on GOG if you are committed to support the GOG version as well (within reason; I personally don't demand GOG version to have Steamworks mod integration or achievements or even multiplayer support (if the single-player is good), as long as it is made clear these are not included in the GOG version). So be committed to release also DLCs and updates on GOG too.
2. If, however, you start having second thoughts because the GOG version is selling poorly or you have some other issue with GOG or its customers, you should do two things, in this order:
a) Pull your game out of GOG. Since you have not willing to support the GOG version anymore, you shouldn't be selling it anymore to unsuspecting GOG customers who are not aware of buying an unsupported version of the game.
b) Offer one of the following to any GOG customer that wants it:
- a free Steam key (to those who still want to keep their GOG version; Supraland developer has offered this)
- a refund to those who don't feel like having the Steam version (the game will be removed from these people's GOG accounts)