I mostly think that your points here, while correct, miss the forest for the trees.
1. Steam's cut isn't 30%; it's 30% on keys sold through Steam and 0% on keys generated to be sold elsewhere. Most games sell about 1/3rd of their Steam copies via external keys on sites like Humble, GreenManGaming, Nuuvem, WinGameStore, directly on their websites, etc. Valve's implied cut would be from 15-20% for most of these games. Of course, publishers and developers voluntarily give up some of the cut Valve doesn't take to those intermediaries, which is another knock against the claim that Valve is an exploiter. It seems hard to argue that in a world where a game is sold for $60 and GreenManGaming takes $6, passes on a $12 discount to the consumer, Valve takes $0, and the publisher takes $42, that Valve is the one exploiting the publisher, despite giving the publisher free keys.
Why does Valve do this? To maximize lock-in in their ecosystem and make money on in-app purchases. They have selfish interests at mind, but those manifest through giving away some of their cut on purchases.
2. Valve also has a fee reduction program that's the exact opposite of Apple/Google. Apple/Google charges 15% up to $1 million revenue and then charge you the full amount. Valve charges progressively less after $1 million revenue (down as low as 20%). I think this reflects closed/open systems;
Valve is worried about publishers like EA (who left Steam and then came back hat in hand because their own service was an expensive and technically poor failure), Ubisoft (who left Steam and are currently bilking Epic for upfront cash while pushing their own UPlay service), Bethesda/MS (who flirt with Beth.net/Microsoft Games Store but both ultimately returned to Steam in the end), Activision (have abandoned Steam for Battle.net). So they want to make it easier for big publishers to come back to them. Whereas Apple wants to maximize revenue off big clients and maximize the PR benefits of helping small businesses. But the point is that Apple, Google, and Valve alike actually do have flexibility in the % they take.
3. Finally, I think the Epic active user numbers are really not super useful. The vast majority of those Epic users are simply Fortnite players (and it's actually a little unclear whether Epic is counting non-PC Fortnite players who sign it under Epic accounts). Having acquired Rocket League, they now can use Rocket League players in the same way. Having acquired Fall Guys, they will likely do the same there in the coming years.
The actual number of users of the Epic Games Store are not trivial, it'd definitely be a strong second having overtaken GOG and Itch and other competitors. But they're likely in the single digit millions.
This in part reflects that the Epic Games Store is not very good. The client does not support basic features, nor does the store interface. Their onboarding process is entirely manual, which they characterize as giving them a "curated" game library but actually it means there's just not that much variety on the store and almost nothing at the low end. They have sales where they eat huge losses to give users discounts, but these are not appreciably better than the kinds of sales that occur on Steam and in the broader ecosystem.
Crucially, to your point that Steam might be over "overtaken" by Epic is that it's not clear there's any competition at all. Every game that released on Epic and not Steam did so because Epic signed exclusivity contracts with the game (typically 12 month exclusivity contracts). It's predictable that all of these exclusives, bar one -- World War Z, have released on Steam within a month or so of their exclusivity expiring. Most seem to report doing better on Steam. Most developers that release on Epic and Steam, without a formal exclusivity deal, are developers who signed "free game" deals with Epic where Epic bulk-pays developers upfront in exchange for allowing every Epic user to redeem the game for free. Epic reports a relatively low conversion rate from free game redeemers to buyers.
All of this suggests that the Epic Game Store, while putting up impressive numbers, isn't really peeling off developers or users because of its lowers cut. I could imagine a world where Valve/Steam are materially hurt if Epic greatly extends their exclusivity contracts, losing billions of dollars specifically to destroy Steam. But I don't see Epic organically overtaking Steam.
My main point here is that the complexities of what's being offered and what's being charged here are obscured when we just say "You ate one third of my pizza! But I wanted that pizza!"