Epic Games Store is offering developers 100% of revenue for six months of exclusivity
(www.videogameschronicle.com)
I think you're missing a big thing here.
Let's take your scenario where there's stores a with 99.9% and store b with 1%
A developer of a game can list their game on store a and store b. Store b will have to compete with higher revenue share. But that would make it enticing for developers to dual list.
-- Legally epic is not forcing anyone to take their exclusive at the offers, but I as a customer realize they're removing choice from me as a customer. I consider that a personal affront to my agency. And I will not do business with a company who I consider as a negative, removing my choices.
If epic just offered a better cut to developers, nobody would have an issue of them. It's the exclusivity trying to fragment the space that makes it annoying and to be avoided