I love Vimperator - unfortunately, it's incompatible with e10s (multiprocess Firefox) and it is also completely dependent on the deprecated XUL. Replacing that would essentially be a ground-up rewrite[0].
It looks like Vimperator is likely not going to be updated to work with newer versions of Firefox, which is a real shame, because it works far better than any alternative I've found for either Firefox or Chrome. I even made my own with the new Firefox API to see if I could replicate the experience, but it's tough[1]. The WebExtension interface doesn't give the same level of control that XUL did, and without which it's hard to give the same feel of Vimperator.
I have followed the situation fairly closely, and I agree with your assessment. I haven't seen anything that makes me think either Pentadactyl or Vimperator will survive the deprecation of XUL. The most promising vim-like addon is VimFx (followed by the Chrome addons like Vimium and cVim which maybe will work in Firefox one day as WebExtensions -- I try on Nightly every couple months but they don't work so far). The reason I say this is that it has active development and developers committed to the WebExtensions transition (plus has less features so it is easier to port). You can follow the progress here:
It looks like Vimperator is likely not going to be updated to work with newer versions of Firefox, which is a real shame, because it works far better than any alternative I've found for either Firefox or Chrome. I even made my own with the new Firefox API to see if I could replicate the experience, but it's tough[1]. The WebExtension interface doesn't give the same level of control that XUL did, and without which it's hard to give the same feel of Vimperator.
[0] https://github.com/vimperator/vimperator-labs/issues/211
[1] https://github.com/ChimeraCoder/electrovim