I don't think that strictly applies here; the stores don't play a predominant role in marketing the games (at least for popular titles), so they don't have exclusivity over an audience.
I think the relevant question is the extent customers decide not to purchase a game because it's exclusive to an alternative store.
To some extent this is intentional. Epic Game's market research culminated in a decision to include fewer social features, and less visual clutter than Steam.
Epic Games seems to be competing more on the back-end, offering better rates and licensing to developers in return for distribution exclusivity, rather than the front-end with features for users. That said, as a user, I appreciate the minimalism of EGS.
As an old fart, developer and gamer, I don't feel much sympathy for those who are wooed by intentional ignorance.
Personally, I will not buy products without reading user reviews; and I find that most professional reviews aren't worth the bandwidth to download.