While Steam as a gaming platform is facing ever increasing competition, Valve likely aren't too worried given how much Steam has been growing over 2020 and into 2021.
After setting a new concurrent user record of 24,804,148 in December 2020, that was once again smashed on January 2, 2021 with 25,415,080 user accounts online but only 7.4 million were actually in-game (thanks SteamDB). It's not exactly a surprise, with multiple user records for both Steam and games being broken through 2020 with many more staying at home due to the COVID-19 pandemic.
The top dog on Steam remains as Counter-Strike: Global Offensive, Valve's first-person shooter originally released back in 2012. It obviously helps that it went free to play back in late 2018 which helped keep its popularity and since the pandemic really hit, the numbers for CS:GO have repeatedly hit new highs.
It will definitely be interesting to see how the Steam playerbase evolves through 2021, especially if we see Valve release one of those secret projects they teased before. Hopefully one that's more available to everyone and not a VR-only title like Half-Life: Alyx, since the cost of VR is still rather high. Considering they're a private company though, they can do and work on whatever they want.