Today... it's Roblox vs everything else. Steam has its data at https://store.steampowered.com/charts/ - in the past 3 days, 45 million peak online at once.
Roblox has 150 million daily active users and 380 million monthly active users. https://brands.roblox.com/metrics-insights. Different numbers. Back in August it hit 47.3 M concurrent users (Steam's record at that time was 41.2 M).
As those gammers are growing up... they're not switching to other platforms.
Blaming this on AI doesn't feel like most of the story.
Gamers are dying out(*) - Belluar News https://youtu.be/_80DVbedWiI