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.
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.