Since we are once again back to "EGS vs Steam" rather than "What cut makes sense": The best possible outcome of this is that Valve change to something closer to 20/80 and they commit to curating their store so that a narrative game about the awkardness of dating doesn't have "Big Tittty Waifus 62: They Are Totally 9000 Years Old" advertised on the store page since both have "nudity" tags.
No percentage is objectively correct. It's a fight between publishers and their distributors. It's totally arbitrary for random posters (myself included) to say 70/30 is somehow optimal or 80/20 is more fair, or actually it should be 88/12, etc. The percent is based on market value, not fairness, and at any rate we're in no position to say what's fair.
I'm not against Epic giving a higher percent to publishers and I'm not against Valve keeping more of the revenue. I care about having a good experience playing games on PC. Steam gives me the best experience so that's where I buy games.
I don't/won't buy games on EGS because it's a bad service, and I resent being "forced" to play games I'd otherwise buy on Steam on their bad service. Publishers who make exclusive deals with EGS have decided they'd rather take a short term payday than give their customers the best experience. That reflects badly on them as well.
If at some point down the line, EGS evolves into a good service, I'll reevaluate. Doesn't look like it will happen any time soon.