There is no marketshare PC game launchers. What even does that mean? The most successfull games are not even on Steam, most played games are LoL, Roblox, Fortnite, Minecraft and besides that, there are plenty of alternative shops every user can freely choose from. Nobody is forced to use Steam, its an open platform. Every single game they buy can be purchased on any of these platforms. You don't even need to buy games on Steam, to buy Steam game keys.

https://en.wikipedia.org/wiki/Monopoly:

A monopoly (from Greek μόνος, mónos, 'single, alone' and πωλεῖν, pōleîn, 'to sell') is a market in which one person or company is the only supplier of a particular good or service.[1] A monopoly is characterized by a lack of economic competition to produce a particular thing, a lack of viable substitute goods, and the possibility of a high monopoly price well above the seller's marginal cost that leads to a high monopoly profit.

Therefore Steam is not a true monopoly. They just have a huge market leader position.