I totally agree that in Computer Science there are established paradigms, but I think sometimes these paradigms keep people from exploring different ways of thinking.

Talking specifically about TermKit: If I'm on a Mac using Terminal to access the local shell, why would I care if the features are coming from the shell itself or the Terminal app?

Of course it would be 'better' or more 'proper' if all the functions were built into the shell, but like you say, it takes widespread adoption. I don't see what the problem is with starting to add features to your terminal and then let the good ones trickle back down into the shell as they become widely adopted.

Seems like a confusing nightmare, but I'm not fundamentally against it.