You just said you aren't arguing semantics but then said its a literally monopoly. That's not semantics - you're literally (literally) calling it a monopoly when it's not.

Marketshare is large because Steam does it best - its often the cheapest, it has ease of access and is popular.

So no, it's not a monopoly 'however you want to quantify it'. Because you can't just call something it's not, you can't just change the definition to suit your argument.

It literally isn't a monopoly and nearly every single game that releases has options. If YOU CHOOSE where to buy a game, that's YOUR decision. It is NOT decided by a monopoly. If all games were only released on steam, that's a monopoly.

Steam do NOT require exclusivity to host content. Developers choose them because they are the best option. The fact we have GOG, Uplay, EPIC, Origin etc etc mean there are Tons of options yet people will parrot the 'steam has marketshare' argument when it's completely irrelevant