As I've said in previous posts Sigil isn't dead. It's recently had a lack of contributors making development stalled. As long as people are willing to contribute to Sigil it will continue to evolve and move forward.
As for contributing myself I simply don't have 20-30 hours a week to devote to writing code for Sigil. That doesn't mean I've stopped writing code for Sigil and as I've said previously I'm willing to provide guidance to anyone who wants to contribute.
Basically, one year go the there were no outside contributors. So for one year not much has happened. A small maintenance release happened. Bug fixes were committed but few and far between. Nothing major feature wise.
Now that all changed about a month ago when Kevin Hendricks decided to invest his time in some bug fixes. Then he decided to start working on a plugin framework for Sigil. He's been spearheading the effort to get this feature implemented. It's not ready yet but it's coming a long nicely.
I know I don't want to see it die but there is only so much one person can do alone. So the moral of the story is, Sigil isn't dead and as long as there are contributors it won't die and it hasn't yet.