I mean, you literally agreed with me. Some gamers are mad that the game isn't on Steam. And I agree. That's what this is about. As for why they couldn't simply do a Day 1 release on all platforms: because that would guarantee that Steam gets the overwhelming majority of sales which would in turn undermine the devs' efforts to protest Valve's price gouging. The game got moved from Steam (pre-existing pre-orders aside) because of Valve's behaviour.
Read my post again.
Gamers are mad because of the bait and switch tactics.
Gamers are mad because of the threats.
Developers and Publishers are more than welcomed to use such tactics... But then my wallet will close... And they don't have a leg to stand on.
I haven't really found any specific sales data. The only figures seem to compare its sales to Battlefield 1, which is one of the best received Battlefield games and the best selling. If years of being the most hated games company and supposed boycotts still cause a game to be sold in millions I doubt a supposed boycott of a semi popular game will do any damage.
In fact I believe this move will make the Epic games store a hell of a lot more popular and will create a serious alternative to Steam in the coming years.