... Except that you don't know what the situation would be like if Epic wasn't able to open an app store -- because steam didn't prevent them from doing so. I get what you're saying, but the fact that steam has better deals than Epic is fairly weak evidence against a competitive market.

Furthermore, steam never had close to a monopoly like the phone app stores have. Steam has always competed with direct sales, publisher stores, brick and mortar, and the windows store, among others. The fact that one more store didn't make a huge difference isn't surprising because the market was already operating properly.