What about the UX for managing all these keys? Every time you try a new app you must create a new key using your master key, so where does that master key live?

In an offline hardware device?, then it will be incredibly hard and only 5 people in the world will do it, everybody else will just paste nsecs and the protocol will be bloated for no reason.

Or in an online device or server that keeps running 24/7 and answering requests for creating new keys somehow? Now we have just recreated NIP-46 but 100 times worse.