Your "source" is an image that doesn't even differentiate between various stores and lists everything as blue, with text that says "steam revenue is probabaly x billion". I couldn't find anything except other similar images when looking up Pelham Smithers reports. In reports from previous years steam is not mentioned. I will rephrase it then if you dislike the word I am using: Steam has a dominant position in the pc gaming market and uses it to their benefit which doesn't (in my opinion) coincide with consumer benefit, also their app is shit.
P.S.: I've used the word monopoly once
So much this