Exactly. Why would you want a single app that has other apps? You could easily go download a standalone app from the app store that delivers a superior experience. There would need to be some competitive advantage or economy of scale of the single app. But what that advantage is remains to be seen.

Just as an example: both Facebook and Snapchat (and I'm sure others) have tried adding games to their platform. Both efforts have failed because you can get a better gaming experience by playing a standalone game. If you want to play with a friend, you can message them and say "hey, let's play X." There is limited upside to playing in Facebook or Snapchat, and a lot of downsides (limited catalog, performance restrictions, etc.)

One exception to this is any feature that requires exposure to an audience. Social media is very good at exposing people to content. Something like Facebook marketplace can be successful because sellers want their listings to be seen by people and buyers want to go to places that have the most sellers.

This is true, today. But look at things like the Digital Markets Act in the EU; there is a significant chance those "super apps" iOS and Android are forced to submit to government regulation in the future, which will create new opportunities - if/once the App store monopoly is removed for example, the ability for any app to become the "super-app" is greatly enhanced, even if very hard. Twitter as brand and platform I could imagine playing in a post-mandatory App store landscape - but so too will a lot of others of course.