An ILLEGAL monopoly (in the USA) requires a person/organization to use anti-competitive measures to attain and/or retain monopolistic control over an industry. This can include predatory pricing, predatory mergers, intentionally driving competitors out of business, and the refusal to deal in normal circumstances.
Steam is a LEGAL monopoly that gained its market share by getting in early and being consistently foresightful, considerate of their customers' needs, and competitive in the industry.
So why is an article at all? Because of the distribution fees. You pay two ways to distribute a game on Steam. First, you pay a one-time $100, but you get that back after $1,000 in sales. Second, You pay a 30% commission on all sales, but that drops to 25% after you bring in $10 million and 20% after $50 million.
Is that unfair? Probably not. Consider how much it would cost to print a bunch of CDs/DVDs, boxes, get stores to carry your game, etc. And if you don't want to do that (no one does), compare it to the cost to distribute your game elsewhere and what features that system has (or more likely, doesn't have).
GOG, Microsoft, XBox, Playstation, Nintendo, has no one-time fee, but has a similar revenue share at 30%.
EPIC Games has a $100 listing fee but has the lowest revenue share I could find at 12%.
So Steam is comparably priced, has a MASSIVE marketshare, isn't attempting to absorb smaller competitors, and they provide a great, stable product.
So is there a problem?