Maybe they don't care if they can snipe customers from the big AAA console and PC platforms? I know I already have tons of games across multiple platforms. Tons of Steam games, a couple Microsoft store PC titles, GOG, Uplay, Origin... and I also own a bunch of games on the App Store for macOS and iOS.
I doubt they'll ever replace the big names, but they'll carve a niche for some people, and there are still exclusives that those people may find worthy of spending a meager $4.99/month, which grants access to a full library of sometimes regularly updated games, and their saves usually synchronize between iOS devices and macOS.
With this hugely fragmented market, no one competitor is likely to completely replace any of the others. Then again, I don't think there simply being a Steam monopoly is a really good thing, either, no matter how nice their feature set is.