How is the 30% valve takes "extraordinarily high"? It's the same MINIMUM that retail stores take on any product, and most other digital storefronts use the same percentage. Compared to Epic, MAYBE they have a MINOR amount of traction, but that's because Epic is doing that to court devs since they're still hopelessly behind Valve/Steam in terms of what their storefront offers.
Want to be on the biggest digital gaming store that has 10x the features (maybe you personally use them, maybe you don't, but the fact remains they HAVE them) of any of the other stores? Well then I guess you'll have to deal with the industry-standard 30%. COULD it be less? Sure, sure it could. So could ANYTHING sold by a retailer. But they aren't. Boo-hoo, that's reality.
Valve isn't ONLY in the position they're in because they were the first; they're there because their store is the best one, period. GoG is in second place, with the unique "no DRM" feature, but that cuts into what they can offer, and they again still don't come close to the number of useful features Steam has.