I suppose lack of competition is the key here, my understanding has been that there is no competition because the reigning business buys out the competitors or uses their market power to keep others out. Not because other competitors exist and just happen to be worse (be it from youth or poor management).

With Epic's attempt at making strides and with Gamepass being cross platform, I think it's fair to say there is competition that exists but it's being resisted by consumers because they are setting terrible precedents. Still, plenty of people use those services, and for the most part key selling sites do have other options available. If it's a Humble pack, generally there is a Steam Key and the publisher key, or more recently an Epic Key.

Steam just has both its age and its value as a service that keeps it popular, but I guess I just don't think that market dominance is a monopoly. You do raise valid points. I'm definitely not trying to be a Valve defender by any means either, they're a big corp that is capable of pulling some bullshit, just that the definition of monopoly I learned has the distinction of leading market dominance with no competition due to anticompetitive practices - purchasing smaller competitors, larger corporate mergers. Not when a business is a market leader because other companies aren't as good. But things change and it's been a few years since I've been in the business side of things.