> Most importantly for our conversation, the Fn key was resolved internally inside the keyboard

That's the worst part about Fn, limiting user customization and wasting keyboard space. Good that this was partially dialed back, but bad that Apple added another exclusivity barrier breaking external keyboards.

> What if Apple at some point decides that Esc means something, and you already used it for something else?

You continue to use it for something else? How is it different from any other default shortcut you don't like and change?

> It’s just a modifier key.

That should be the end game! No lock in, no weird limitations like "cannot map Mission Control to ↑"

There is no hope for Apple to make anything good out of it (⌃⌘X is their peak ergonomic design), but at least you'd be able to freely use the key yourself