I bet you won't elaborate, as I don't see how that is better for us when the games cost the same or more vs. Steam and we don't even get the per-game forums to help solve technical issues.
I how exactly is it better for the devs (not publishers) who already finished working on RDR2 with their 100 hour work weeks? Will they get bonuses because of the 12% vs 30% cut on Epic? No.
Simple economy, just think it through for a moment. Put yourself in the position of project planning and making an investment/deciding on budget.
Option A:
Distributor takes 30% of the cake.
Option B:
Distributor takes 12% of the cake.
If you invest 10 million, that is 3 million to Valve, or 1.2 million to Epic. That is 1.8 million to spend EXTRA on content, marketing, features, or hiring new devs. Or it means investing perhaps not 10, but 9 million, with 1 million ready to spend on a different project.
You applying this to existing games on the market makes no sense. This is a long term thing, not something you will see the effects of today.