I understand that is important to you, but as a company/studio, we have to prioritize. For launch, there is no way we will have additional platforms, as Tinto wants to focus on having a solid release on Steam and it is where the huge majority of our players are and all the features are well know and available to be integrated with the game.
To go on GOG, or any other platform, we need to be in a position where we see the potential to have more players experiencing our games, while not adding an increased burden in our operation. Mind you, we do not release and forget a new game. We plan to support it for 5, 10 years, with free patches and DLCs. And every single platform we add, it is another thing to make sure we integrate it correctly with their features, check for bugs, do all the release steps every single time.
We do have other GSGs on GOG, but without sharing specific numbers, I can say that the number of players using it on a monthly basis is very very small compared to Steam. I understand that the studio wants to focus on getting the game out there, make sure it is doing great, before evaluating adding more layers in the operation. And from a business point of view, this is not a priority either.
All that I said here is the same rationale we use for other things like languages, MAC/Linux conversations, console, etc. Every single decision needs to account for years of support, additional point of failures and additional steps on every single thing we do, and then we compare to what is the expected return of that investment (both in numbers, but also in opportunity costs).
So yeah, I would never say "we are never releasing on other platforms", but I don't want to do any promises at this point. Let's get this game out, make sure it is great and then we see what the future has for us.