>If this was truly that simple, don't you think someone would have already done it?

Someone already has done it. An one-man-shop built Sublime Text by himself.

Surely it's not having JS as an extension language as opposed to Python (the only difference between ST and what I propose) that's making it difficult.

>Furthermore, is there even an actively maintained, open source, cross platform, development-focused text editor with native UI components?

There's a "cross platform, development-focused text editor with native UI component" that's working great.

Whether there's an open source editor like that is orthogonal, since what I'm discussing is whether is technically feasible to create such a browser -- and the existence of ST proves that it is (and that's by a single developer: with resources such as that GitHub or even better MS, have, it would be much easier).

The choice of license is just a decision after that.