This is kind of how things used to be when you had 3rd party clients for things like email/irc/XMPP. Eventually it was decided that having a unified design and feature set was much more beneficial and simple for users than being able to theme the client.

I agree. Six years ago during COVID I wrote a document describing my idea of a dream personal computing environment, where all functionality is accessible using an API, enabling scripting and customizable UIs. UIs are simply shells covering functionality provided by various objects.

Unfortunately I haven't had the time to implement this vision, but Smalltalk environments such as Squeak and Pharo appear to be great environments to play around with such ideas, since everything is a live object.

> Someone should be able to freeze their user interface in time if they wish.

Why?