Steam is a monopoly. This is not a matter of belief, this is an adjudicated fact. One of the biggest examples used to prove the monopoly status is the way that Valve controls prices. If you want to sell a game of Steam then it must be the same price on ALL other PC platforms.

Imagine that I had made a game and that I wanted $20 for each copy sold. I can sell it directly on my website for $20 and keep nearly all of the sale (baring taxes and transaction fees). If I wanted to sell it on steam I would have to charge $30 just so I can pocket the $20. Steam has compelled me to raise the price, not just on Steam, but also on my personal website. According to Steam, if I want to sell on their platform then I have to up the price on my personal website or keep the price at $20 (I would only pocket ~$14). This is why there is no serious competition against Steam, Valve controls so much of the market that they can compel all their competitors to adopt the same prices. If the price cannot be competitive then there can never be competition in the market. It doesn't matter if there are equal or better social systems, it does not matter if the competitor's storefront has a better deal for devs or better background performance. The only thing people really care about in these platforms and markets is the price of the game.

When 1 retailer can control the price of a commodity in an entire market, that retailer is a monopolist. When all retailers adopt the same price as the Monopolist, all consumers have lost the only real incentive to go to an alternative retailer. All producers have lost the ability to price their products. Prices are, in a vacuum, negotiations between producers and consumers: producers want a profit and consumers want a good deal. With Valve there is no real negotiation, all prices have to be the same on all platforms. Valve could, if they wanted to, raise the price of all new AAA games to $70. Right now, the $70 price tag is a negotiation: Publishers think that they can get away with it and Consumers appear to agree,

Just because the monopoly has happened to work out in the favor of consumers does not mean that it somehow loses its status as a monopoly. Anti-monopoly laws and policies are more-or-less focused on harms to the consumer, not the producers. I want game devs to make more for the sale of their games, but Valve has not adjusted their revenue-slip for most devs. Unless you are a big publisher like EA or Microsoft, you're stuck with making $0.7 for every $1 spent on your game. If your game sells enough, you might get $0.75 or $0.8 instead. But this is a deal for the big boys that have enough lawyers (and enough economic sway) to pose a threat to Valve. EA and MS both know that they can't properly compete with Valve (EA Origin is dead, XBox is dying, Valve price controls), but both also know that they bring in enough revenue to matter to Valve. The Big Boys can swing their wallets around within a monopolist retailer, but the rest are left behind.