i.e. a framework.
> Then because they're complicated they're difficult to learn.
It's easier to learn a publicly available framework, as there is lots of help on Stackoverflow, and usually the popular frameworks have good, or just existing, documentation.
> existing code isn't dependent on some obsolete framework - Ember/Angular 1/etc.
The "app architecture" can also become obsolete. But now instead of a migration path, you're stuck with the person who made the "app architecture" most likely having left the company.