also it didn't particularly make sense on its own terms. Even if I am committed to pricing the same as on Steam, I still get to keep that extra 30% on the sales that I do make.

And it's not as though the sum total of marketing you do for a game is stick it up on Steam and wait for the recommendations to pour in. But still most game promos and ads I see include a Steam link rather than one to the studio's store.

There's over 100,000 games on Steam now, so the notion that you're relying them to market your game for you is highly suspect.