A monopoly is when a commodity only has one supplier. From a legal perspective being "close enough" to 100% market share is enough to trigger protections against it. As luck would have it, Steam is not even remotely close to being the only supplier of videogames on the PC platform. Please consider the following games that are available on PC, but not available on Steam:
- Fortnite, one of the hottest games in the world.
- Minecraft, a generation defining cultural touchstone.
- League of Legends, the biggest Moba in the world and much larger than Dota 2.
- Every single game from Electronic Arts since ~2012, one of the largest publishers in the world, including series such as Battlefield, Battlefront, The Sims, Mass Effect, Dragon Age, and Apex Legends, the latest hotness taking the world by storm.
- Every single game from Blizzard, including such minor titles as World of Warcraft, Overwatch, Diablo III and Starcraft 2.
- Several recent major games from Activision, which along with Blizard are the largest Western publisher. Call of Duty: BLOPS 4 and Destiny 2 were not on steam and most of their major games going forwards probably won't be either.
- An increasing number of Microsoft first party titles starting in about ~2016, which are exclusive to the Microsoft store. Every major release this year onward that is internally developed by the (increasingly substantial) Microsoft Studios teams will be Win store exclusive.
Then there are publishers who are flirting with leaving steam and going to do their own thing. Ubisoft is doing a uPlay + Epic release for The Division 2 and damned Epic with faint praise because the move resulted in a dramatic increase in the proportion of preorders being on uPlay (compared to when it was on Steam where people wanted to buy the Steam copy generally). Ubisoft will probably move away from Steam towards a uPlay exclsive model eventually, with small Epic store releases for some games, Steam releases for other games in the interim, decided on a per-game basis. Once people show they're ok with uPlay exclusives, it's inevitable they will just switch to taking 100% of the cut for themselves. Why wouldn't they? Ubisoft, along with Activision-Blizzard and Electronic Arts make up the top 3 western publishers.
(Side note: Tencent, the largest videogame publisher in the world, who owns a majority stake in Epic games, also owns a 5% stake in Ubisoft and may seek to increase their holding now that Vivendi has dropped their takeover bid and begun selling off its stake).
Bethesda, another major Western publisher, has begun experimenting with going exclusive to their launcher. Fallout 76 remains a Bethesda Launcher exclusive, and Rage 2 will be going exclusive. Although not yet explicitly confirmed, Doom Eternal and Wolfenstein: Youngbloods are extremely likely to be Bethesda launcher exclusive games.
Steam will be enjoying only a minority of AAA game releases on PC this year, as well as a decreasing percentage of AA game releases on their platform. From western publishers, the last major holdouts are Take 2 (Rockstar social club exists and they could easily pivot to their own launcher if they really wanted to) and Warner Brothers Interactive (too few games to make it worthwhile currently). Even mid tier people like Paradox interactive have their own launcher and storefront which they may some day pivot to. Japanese publishers are currently
far more onboard with Steam, with Capcom being the golden child this year, but Square Enix, Sega and mid-tier Japanese publishers are still showing an interest in the platform.
Even before Epic came along and started signing exclusivity deals for games, Steam has been slowly loosing major publisher support over a period of years as more and more companies decided that they wanted to keep 100% of their sales even if that meant a decline in total sales volume for certain titles. Epic are not overthrowing a vast monopoly, that's a fallacious yet oddly popular misreading of the industry situation. Rather, they've sensed blood in the water and are pouncing to focus on the most consistent section of the market that Steam does have - Indie developers smaller scale games who rely on Steam not just for distribution, but often to find their audience as well.
If Steam had a "monopoly", then it's monopoly was on a subset (smaller / indie games) of a subset (PC games) of videogames. And even then, only under the loose definition of monpoly meaning "market leader who you would be foolish not to ship your game on because there are so many customers on the platform".