De gustibus et coloribus non est disputandum.

If you're working on a small game project without an existing framework/platform, you usually build a really bargain-basement game editor alongside it. That editor takes a long time, and it's often spent in arranging UI stuff.

If you're already using Blender to make assets, there's some benefit to stuffing Blender's UI into your game editor too.

Imagine this spinning off and becoming this generation's GTK.

Why not? For similar applications it would be really beneficial to not have to reimplement all the little UI interactions they've polished over the years

How about a simpler UI consisting of just a text input field for AI.

More like a “why not both” situation, imho. I have apps I could port to use this immediately.

Most native widget toolkits don't actually support rendering into 3d canvasses very well, nor very compact UI layouts. I generally prefer native widgets, but for a dense prosumer application, they aren't always the correct choice (and you'll see even Apple stray from their native UI widgets for, say, Logic Pro)