[jcranmer]

Trying to dig into the sources of that article a little bit, what makes the most sense to me is that the $4.3 billion number is Steam's cut of the sales. If you go for a 3-4× multiplier, then you're looking at somewhere around half the total PC gaming market going to Steam.

Which kind of feels about right: there are other game stores; Ubisoft and Origin are the ones that cater to AAA publishers as well. Not to mention that things like the Microsoft Store could well cater to the surprisingly large casual game market (one of the sources quotes $5.2 billion for "Browser PC Games", which I think is reflective of how big casual games are). And you can find people who publish and distribute games outside of Steam.

So saying that Steam has a dominant but not overwhelming market feels correct. The numbers I see most bandied about are 50-75%, which look to be estimated from very old data from what I see. I'm sure if any competitor to Steam had reason to believe they were larger than Steam, they'd be trumpeting it as loudly as they could.