KetobaK: It's not profitable for a store to get the 12% because even if the distribution expenses disappeared in the digital era, the cost of servers, salaries, enhancements and all the aditional features of the store still are present. Stores no only are in chage of sell the game, but they also promote and organise sales, digital stores also have great expenses so 30% seems really fair for me.
Not necessarily. A lot comes down to the volume of sales and the split of fixed vs variable costs on a sale. Unless you're in finance/senior management at Valve or GoG, you have no way of knowing whether 30% is fair, a tight margin or profiteering. The split that the store takes HAS to cover the costs of distributing the game to the user in full (which effectively means an apportioned server/storage space and the average costs of transmission of the game to the user). It then also needs to contribute to the fixed costs of the business - which includes your other SG&A costs, as well as taxes and leave investors with more profit than they would have made by investing their money in a savings account. It's a rational decision to take a lower margin if it attracts enough additional developers to sell enough more units to cover the lower cut that you're taking.
In general, Epic has been a good thing for PC gaming because it's challenging the hegemony of Steam in a way that GoG has been unable to do - yes, it's another DRM platform, but at least it's fostering competition. Remember, exclusivity doesn't mean a monopoly - the games market is wider than that. It's not a monopoly if you can only buy Colin the Caterpillar from Waitrose as other Caterpillar cakes are available in other supermarkets.
In response to OP's question, to my mind it depends on a) whether GoG can make sufficient additional sales for it to be worthwhile - which would mean publishers committing to DRM free and b) whether the trend towards 12% is causing this to be a norm, without which people wouldn't bring games to GoG anymore if they didn't change.