Yeah I agree with you. Some games will be available only on steam whilst some will be available only on the Epic Games Store, and this means that buyers will invest in both of them. Though as for the 88% revenue for the developers, I think it's great. Steam will try to match with this, so far they have already put something in place: https://www.engadget.com/2018/12/01/steam-gives-successful-developers-a-larger-cut-of-game-revenue/
But Epic Games was the first to make that change and that means a lot. I feel like most developers (who mainly use UE4) will now switch to Epic Games.
The whole thing was a good move on Epic's behalf and Steam now isn't the only PC gaming giant (at least not until Epic Games Store gets it's footing.) I hope it doesn't break out into war, and they can exist in peace like Steam and Humble and GOG.
As for me most of my games are on Steam, though I didn't have Subnautica on Steam so I picked that up and have already started my Epic Games library.
RE: Epic Games (creators of Fortnite) and their 'Epic Games Store'