I don't think you've read any of the thread if you believe this.
A) They're not losing millions of sales. The number of people that REFUSE to buy anything not on steam is miniscule and only getting smaller over time, not bigger.
B) They don't require 'compensation' in the sense you're trying to paint it, they get flatly more money on the Epic store. It's not a moneyhat, it's a business decision based on Epic offering them more than Steam.
Now that Epic is putting in 2 of the reasons everyone in this thread crying keeps bringing up for why their store sucks and they will ONLY buy steam products (even people that don't live in regions that need regional pricing), the reasons to remain only with steam are slowly shrinking to "Because I'm a PC gamer that wants to be a console warrior at heart" and that's gonna be largely it.
You underestimate the amount of people that buy games because they are on sale, on the front page, or just one click away from an impulse purchase while you are already in the launcher.
Sales: - Epic already said, they are not big on Sales and you must first open the EGS to go to the sale.
Front Page: - if you don't open the epic launcher, you don't see the front page and you don't see that game xyz is out now or be reminded that it came out 3 months ago and you wanted to try it out.
Impulse purchases: You can't impulse buy if you are not in the launcher.
Yes, many many people will buy the game regardless, but don't underestimate the tens of thousands and in the case of big publishers, the hundreds of thousands of impulse buyers/late buyers/sale buyers.
Also: don't underestimate the reluctance of people to add purchase information to yet another storefront. Do you know how many people still only buy games through the adding of Steam gift-cards? PSN and xbox-live gift-cards are always bestsellers on Amazon. If the EGS doesn't have gift-cards, they lose a bunch of customers who only buy games through gift cards (especially in specific countries outside US/EU, where game purchases through gift cards are the norm)
BTW: Do Fortnite players even see the front page of the EGS when they start the game?