You're replying to a chain about how valve got steam started, are you suggesting that the market was existing in a state of monopoly prior to their actual existence of the product you say held that monopoly?
Not really. When Steam was the only digital distribution marketplace in existence, it didn't have to be better than any other digital distribution marketplace to convince users to switch to it, by virtue of being first. The followers who look at Steam's profits and want to capture that for themselves have to do so. It just happens that it seems every marketplace since has attempted to compete for developers (particularly in the vein of trying to achieve exclusive games) and generally ignored competition for users except as a byproduct of developer competition.
And the result is that users overwhelmingly prefer to use Steam, with alternatives largely relegated to at best grudging acceptance for those games that require alternative launchers. Since companies are reluctant to post numbers, it's hard to tell what the exact situation other than "Steam is well over 50% of the market", but the next largest is probably GoG, especially if you exclude self-publishing from statistics (if you include it, the popularity of Fortnite might push Epic Game Store into second place). And note that GoG is pretty much the only store that offers users a specific value proposition to use them over Steam: GoG is DRM-free (better publisher/distributor split is a value proposition for developers, not users).
