I think you got the whole thing mixed up. Sure Valve takes a huge cut, but if game does poorly Valve earns less as well. So there’s an incentive from both parties to make sure game succeeds. But in the end Valve makes sure you as a consumer get your money’s worth, hence why they even added no questions asked refund policy. Policy which has resulted in more purchases than before, because risk of not liking the game is non-existent now.
Epic on the other hand is forcing users to buy into their ecosystem by way of exclusives. Developers use this to make sure project succeeds even if it’s not good. That is to say they get the money regardless. But this model is not sustainable as Epic has to earn money at some point so number of exclusives will be lower and lower. At the same time they are encouraging developers to not try as hard to polish the game since they get the money regardless.
Fundamentally approaches are completely different and Steam’s approach can’t fail because they cater to customer while Epic is just trying to force people away while offering subpar service. And whoever holds the money holds the power.