This is totally what atproto offers, see my peer comment to yours, then come back and read this.
I'll add here that there are a bunch of experiments going on which aim to break down apps into features. One example is DMs. Ideally all apps can use the same DM infrastructure (MLS based) and as a users, my DMs are the same in any app as my dedicated messenger app. Many people have had the idea to build a "browser" and any app can use bits of another (leaflet blog publish creates bsky post). Some cool experiments around things that look like web components, where you can create records that express a way to render something.
Another place user vs app choice comes into play is around the graph itself. One of the early dreams of app builders was that there is this existing network and you don't have to bootstrap a social network from scratch. While this is partially true, it does turn out reaching network effect is not so easy. Around this, there is a multi-camp debate on whether apps should reuse social graphs or not, one specific example of this is should an app automatically, upon request, or not at all: start from a user's existing social graph.