it depends on your definition of monopoly. For example the US FTC classifies a monopoly as a company with significant and durable market power with the long term ability to raise price or exclude competitors.
Steam would definitely meet that criteria, if you aren't on steam your game is very unlikely to go anywhere. Can it? for sure but it's significantly less likely to be successful, and steam basically sets the standard for what should be on a storefront and pricing for deals.
Being said, the act of being a monopoly in the eyes of the FTC isn't a bad thing either, as long as the position isn't being abused, which Steam currently is not.