Regarding Storming Kiwi's concern, much of this hinges on how you believe people make purchasing decisions. Do they (1) See a game at a store and then buy the game only because it's at the store they like, (2) See a game they have never heard of at a store they like, investigate the game, and then buy it if it looks interesting, or (3) Learn about a game, decide they want it, and then go to the store selling it to buy it?
I'm not a computer game marketing expert, but my guess is that most people learn of interesting games through channels other than game stores and become interested in it that way, so I think (3) is the primary vehicle that drives sales. However, option (2) which has a marketing component is also important. Presumably option (2) will be covered when the game shows up on the Steam store with some fresh marketing.
Personally, I would prefer to purchase the game over Steam, and ironically wanting to purchase Rebellion is how I ended up on the Steam platform in the first place. However, I'll probably purchase the game early access at Epic. I already have Epic's platform installed and have obtained a large catalog of free games from them.
I'm not upset about the game being an Epic exclusive for the time being; I understand the reasons for it. It's probably best for us fans to be thankful that Epic Games made this possible (if that is indeed the case) and that a Sins-2 is being produced at all and focus on how we can help the Devs make Sins-2 a better game and how we can get other 4X and RTS players interested in it.