How is this different from other walled garden game systems/game stores like Nintendo's eShop?

Yet they seem to keep older games and apps, even niche titles for a limited audience (such as music production apps or BASIC programming apps) which I greatly appreciate.

I would agree that discoverability isn't great on the eShop, but there are dozens of enthusiast web sites which are pretty good for game discovery (also aggregators like metacritic.)

And, as I noted, I think Apple already has a good approach which they're using with Apple Music - better human curation including category/genre/time period/artist/etc. playlists. Podcasts/radio shows also help.

Many games (at least ones that aren't in the "live service" category) are more akin to movies, music, or books than to some sort of perishable produce, so a curation approach that balances older content with new content makes sense.