I really wish there was a way to get a slimmer GOG client. I liked the idea of Galaxy 2.0 having all my games in one place, but third party launchers like Playnite have far and away surpassed it in terms of speed and useful functionality, and now it's just a bloated mess that feels unintuitive. At this point I just want GOG games on my Galaxy client.
As for them putting EGS games on their store, this seems dumb. There will always be this issue where Epic gets to hawk its wares on GOG but only games GOG doesn't have, so it doesn't really give you much choice - you just get to buy a bunch of shitty Epic exclusives that you couldn't get on GOG anyway. So really, only Epic wins, because this just legitimises them and they sure as shit would never return the favour.
More interesting functionality in a client would be a store selector, which just takes you to the webpage of each store, lets you buy games, maybe have a price comparison plugin built in (i.e. if you're browsing Steam and a key is cheaper elsewhere, get a little warning), and then once you've bought the game it'll auto-redeem it if it's a key and tell the client on your PC to install. Once it's installed it'll then add the game to the launcher like it's one seamless system.
You wouldn't even need to get permission from any of the store owners because it'd just be a webpage in the client, and when installing games it'd just hook into functionality already built into the client. Make it open source like Playnite is now, and job done.