Thank you for the wrap up varlog. I think it was time that the thread would start addressing the quesion "How to make Sigil live" instead of discussing some private fork ideas.
Yes, C++ is not the easiest language to develop in, but it's still the 3rd most popular programming language according to
this. So, I don't think this is the biggest problem.
Having a huge code base, on the other hand, can be very difficult to handle if you don't have a couple of "dedicatged" programmers.
To extend your idea, my suggestion would be to investigate which functions of Sigil can actually be converted into plug-ins. A smaller core would mean a smaller scope to focus and less effort for the core team.
One other option would be to drop cross-platform support and develop it only for GNU/Linux. That, I guess, would also simplify things for the core team.
To keep the project alive, I also think it's also key to spread the word. I suggest posting an "ad" (prepared by user_none, perhaps) to some of the forums that we contribute to to see if anyone would be interested in contributing.