Speaking of other sites and other 3D designers,
How close to here is this proposal?
Very far apart IMO. Keep in mind that the only reason this site is/was ideal is that Blender is a complete package! While other similar software can only model, render etc. We have a Game Engine, a Video Editor, Compositor, Text Editor, Tracker and the list goes on.
There was a previous proposal to this that was called 3D Graphics.SE which died after a month or so. While there might have been one or two Blender questions possibly, IMO modeling and mockup questions alone cannot constitute to keeping a site as dynamic as SE sites are populated and busy. Also people who use those commercial software are usually trained, working professionals who occasionally might have a few questions about this or that but not enough to need an entire site. We are open source and the majority of our user base are hobbyists and or amateurs, plenty of questions to be asked and answered and experience shared.
Here is also another overly broad one for CG Artists which appears to be going nowhere anytime soon.