Most of this should be available on the PinePhone at launch (albeit not as nicely as on Android).
Bank - should have a website (and if they don't have a website they most likely don't have an app anyway)
YouTube - website again (youtube.com)
Skype - has a web app and a Electron app available for Linux (web.skype.com)
Uber - has a PWA that can be used for booking rides (m.uber.com)
Spotify - has a web app and a Electron app available for Linux (open.spotify.com)
In my experience mobile banking is more often than not provided via app exclusively.
Doesn't that require Widevine DRM?
All it really needs to do is make calls and have good browser (Firefox would do). Signal would be a nice bonus.
This might not be enough for all (i am sure it wont), but for me and several other people I know, it would be enough, to be our main phone.
The market you're describing is minuscule.
For traditional CRUD apps, PWAs are quite alright, and one avoids having to deal with Android's always changing "best practices", with a stagnant Android Java.
If the user can't interact with their bank, whatchu YouTube, talk to their friends and parents over Snapchat or Skype, call Uber, play Spotify, etc etc etc, it doesn't matter in the least if your phone is 100% open source.
Microsoft had sunk billions into trying to get the ecosystem off the ground. What makes you think that "a truly open system" will get that ecosystem?