most indies don't (and usually shouldn't, unless mobile) target casual gamers, and they can't bet on "stumbling upon a random game" to sell.

That stuff isn't easy but fortunately Valve isn't the only one offering that, and pretty much every other storefront offers lower rates if that's important.

What I mean by “should” is if game sticker price is $10 and as Indy game dev you can get $7 basically for free if casual stumbles on your game.

I assume such dev can and should also use all other available promotion options of course.

So from my pov 30% is fine to pay. Especially that after initial effort of putting game in.

Sure, but when your game is selling in the double digits, that's not something you can bet on. Discoverability is a multiplier and if an indies' is very low, Valve doesn't help there. You gotta pay $100, so I need at least 14 people to "stumble" and buy my game just to pay that back. Then I need 140 people so I can get that $100 back.

If valve wants to earn those 3 dollars, they need to ensure my game can be discovered to begin with. "becsuse there's a lot of people on steam" doesn't cut it these days. That was valuable at one point but I'd say aroind 2014 or so it ceased to be so.