Update: The Theoretical Physics proposal will not be closed
I can appreciate and understand both sides of the debate (focused vs. broader appeal). That's why I've been watching carefully the Area 51 discussions and the development of Physics SE. This proposal has inched along slowly over 8 months while the Physics Stack Exchange continues to evolve.
At this time, I feel strongly that splitting the field of physics between two separate sites would be materially harmful to both the proposed Theoretical site and the Physics SE. Having two related sites like that will only draw much-needed attention to the detriment of both.
Physics SE has been doing fine in the Area 51 analytics, but there have been concerns of steadily declining traffic and visitors. If Physics SE had the "big city" problems of a Stack Overflow, there might have been merit to splitting up the community. It might makes sense someday, but not now. I don't think either site can take the additional hit on Physic SE's declining traffic.
But traffic isn't the main reason not to split up the physics community. The Theoretical Physics proposal was created when the Physics SE site was very young. At the time, it wasn't clear if the two branches of study would benefit from being on the same site. But the Physics SE has made tremendous progress and, to their credit, this site seems to have successfully and harmoniously integrated these areas of study.
In short, I feel that the scope of questions outlined in this proposal have a home on Physics SE. That doesn't mean that we can never open another physics-based site. There may be a time where that makes sense. But I am planning on closing the Theoretical Physics proposal and wanted to let you know how I came to that decision and to see if there were any further issue I should consider before finally closing the proposal.
Proposal: Theoretical Physics