Just because Epic has people playing Fornite or buying Metro there, it doesn’t mean those same may be interested in anything unrelated.
Fortnite is a social and pop culture phenomenon; people play it because everybody plays it. Metro and Witcher have an established fan base, a good chunk of which would buy the game mostly wherever (Metro controversy notwithstanding, whose practical consequences remain to be seen).
All of these titles share a common trait, they’re all past a critical mass (Fortnite way past) that makes the activation cost low, bordering on zero.
The argument went that the Epic Store ensures survivability for an indie because of the higher cut going to the devs.
I say percentages alone don’t pay bills: it always takes two to dance the bankroll tango, margin per sale, and sales. An EGS exclusive makes more money for the devs as long as it can sell as much as it would on Steam.
For that to happen, one needs activated customers that don’t mind some inconvenience, IOW they must really want the game you’re selling, to begin with, so that platform is a distant second concern.