Epic Games Store as the leading market share does not sound likely. They would have a seat, but in no way would beat out Steam. As it is, Valve and Steam started as more Indie and became the standard for digital market games, even have a partnership with HumbleBundle. Then with Epic Games, they have been around for a very long time with the Unreal Engine, Tencent, and Fortnite. With the massive funding from Fortnite and Tencent, Epic Games' 'worth' can be seen as quite very inflated, especially seeing as how they started with a giant net worth before they opened Epic Games Store.
The idea that Epic Games being a small company does not make sense. Epic Games started in 1991 and Steam started in 2003. Epic Games has Tencent; Valve just have themselves. The major difference is that Steam is a digital market for games while Epic Games made games.
When Epic Games Store came out, they focused Indies and their giant Fortnite fanbase, plus giving away games. This was positive all around for them but the great majority disliked them. As time has gone on, it has been seen as very beneficial for Indies, plus allows for two separate launches: Epic Games Store and Steam. Some even did Early Access on Epic to launch in full on Steam. So, yes, the end game is to be on Steam. The exclusivity of Epic Games Store is what brings drawbacks and as such makes it more of a test if the game can do well since they only accept games that they foresee selling well and give the devs an advance payment. Steam accepts all but doesn't help that much past that, but it is the most popular market.
Steam will most likely stay as the top digital games distributor, mainly off of reputation, deals, connection with humble bundle, and how everyone has it. It is the same scenario as Nintendo: everyone has it and will keep going with it even if something new comes out.