The term "monopoly" is used in economics and laws and they can mean different things but Steam is not a monopoly in either of them:
Steam used to be a monopoly just because they had no rival but they lost their monopoly status a long time ago. Now it's unrealistic to say Steam is a monopoly (economically) because they are not the only power that decides how the entire video game industry should be and they cannot actually do what AAA companies should do in the context of online video game distribution services. That's why Ubisoft and EA sells their own games on their platforms and our fatty Gabby cries about it on his private yacht lol. For Steam to be a monopoly they would need to be the only company sells video games online, however there are already major significant players in the market like Microsoft Store, Epic Games Store and GOG. And then we have Green Man Gaming, Humble Bundle and tons of minor global and local online video game distribution services that can be specific for an entire country like an ISP having a service to sell video games for their customers. We can accept that Steam is a dominant market leader and there are a significant number of players in the market and that's why instead of calling Steam a monopoly we call it a dominant market leader in an oligopolistic market structure and that's why since oligopoly exist in the market there cannot be a monopoly in the market. Easy peasy eggs are tasty lol. What monopoly simply is like there is a huge cake and our fatty Gabby eats the whole cake while other kids can only count how many times fatty Gabby had to chew to eat the entire cake. However what actually happens in the PC market is our fatty Gabby eats 2/3 of the cake while 1/3 is shared among tons of kids and that is what we call oligopoly and that's why we call Steam the dominant cake eater™©℗® in oligopolistic cake eating event lol.
In law being a monopoly is not illegal but monopolization (intentional act to use unlawful ways to be a monopoly) is a crime because when people try to have the monopoly it ruins tons of laws that protect society to make lawful and fair business and it can collapse society and economy that threatens the social life of the entire world for people to actually survive, money can flow and people can find a job. It can lead to another The Great Depression period that forces 99% of society have to dance in front of rich people for 18 hours just for $1. To prevent it companies are protected as much as customers. Since our rule 4 here forbids politics all I can say in the law context is major part of the world actually accuses Valve for using its dominant market leader position to break laws to be a monopoly by unlawfully trying to destroy the established fair and legal oligopoly in the market and therefore currently Steam is accused of trying to be a monopoly in law content. Since my uncle doesn't work in Valve I have no idea if they are really guilty so as far as I know Steam is not a monopoly (illegal one) by law because they are not punished by it (but it seems they likely will be).
When we look at it socially what monopoly means is subjective therefore it is trivial. We cannot decide what objective words and scientific terms means, all you can do is to obey the dictionary lol. Yet we may talk about Steam losing its potential to stay as a dominant market leader because society pukes at the idea of DRM so they demand games to be sold on GOG instead. And then Steam turned into porn shit that it got harder to explore its store to actually find a decent game. And then Steam removed regional pricing, instead they use adjusted $ prices that ruined personal preference of tons of gamers around the world considering how taxes and bank fees may work in their countries and therefore as a result they started to prefer Microsoft Store and Epic Games Store more to not feel like a sucker buying games on Steam when they can buy the same game cheaper on another platform. And then Steam is bad for many indie developers because they ask too much $$$ to sell the game on their store and they ask too much % from every sell you make. Considering the economy suck and Valve doesn't care, people started to prefer itch.io and Epic Games Store more. As a result Steam risks being customer-friendly in the entire world by adjusting itself to be "only for rich people" more than it already is right now. It causes weakness for Steam and it is what Epic Games Store wanna exploit to win hearts of non-rich people in the entire market. Steam started to ignore being a global market, they only focus on Americanos and Europeanons for a long time, and subjectively you can even say Steam's objective status on being a dominant market leader doesn't matter and it has null effect on most of the world where they stopped using Steam long time ago. So subjectively either Microsoft Store, Epic Games Store or even GOG became the "one and only platform" so many people care about anymore. While I do not think Epic Games Store will exist after a few years, if they play their cards right, keep regional pricing, implement no DRM, better social features than Steam has in terms of custom profile and backgrounds for their profile page, way more gamer friendly features to ease streaming games via Youtube, Discord and Twitch, game rent system (you may choose to buy and keep your game but you can also rent your game to play within a time limit something akin to Game Pass), even dating feature to match people according to their gamer profile to make it work like a Tinder shit no one can stop popularity of Epic Games Store!!! lolol
So instead of turning the shit into Korean and Indian drama to play on victim mentality you gotta think as a business-person and accept to make a significant profit Steam should be your top 1 choice to sell your game. As a customer you may pick whatever platform you want, but to make business naturally you will pick what most customers prefer. However it is stupid to just pick Steam, so better pick every platform you may sell your game on because it is important for you to respect the personal preference, financial reality and legal restrains of your customer. So from a business point, for example, EA and Ubisoft not selling their game on Steam can only serve as butchering their potential profit and customers' subjectivity to actually find it okay to buy their games. In every period they don't sell their game on Steam they risk being forgotten, making customers make a stance against them to not buy their games for feeling resentful, and customers may forget they actually exist when people won't waste their time browse every available platform when they can barely browse main page of the Steam to keep up with what new game being released on. For example if you pick Steam but not GOG you will lose die-hard GOG customers who pukes at the idea of DRM, if you pick only GOG but you don't sell the game on Steam your game will be less likely to be known around. In the end how the Steam is and the fact that Steam is a dominant market leader is not a bad thing for you, on the contrary it is good for you to make a profit. You gotta be glad that there is a platform that is actually that popular and PC market is being kept that alive so you have a greater chance to make profit. However yes the way Steam requires too much money to publish your game and it asks too much % from your every profit to actually make a meaningful profit when you are not an AAA company sucks but still it is better choice to make sure your game is on Steam.
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