You want to sell software you wrote to run on an iphone. You have zero choice. Apple tax your revenue.
You want to sell software you wrote to run on a pc. Steam is not your only choice. I am not defending steam or valve here, I've never sold anything using their stuff, nor am I suggesting anything other than that their market power over pc compared to apple's store over the iphone is not remotely comparable.
It actually works against you to suggest apple's iphone software store and steam are comparable at all because it's so incredibly bogus.
You want to make the case that steam suck too but with loads less market power. Go right ahead. We're listening. You don't need absolute and total market power to be abusive of it. Apple will immediately attempt redefine the market to include android or people spending money on coca cola instead of apple product to suggest that customers have real choice so there is no market power abuse here.