@rondarius Thanks for your reply.
I assumed the problem was the work to bring it to a more recent version of Halium (through Android). I thought that maybe if that work is already mostly done and the subsequent effort to bring it to the common mutualized Halium base is lighter than expected, it would make things easier and maybe even manageable for an extra couple years of support.
But from your answer, it seems it's not the only block of work to undertake to keep on supporting it, and kernel updates are another big chunk that amounts to too much.
It would be sad, I kind of thought it was a 1st class citizen (in the device support) and would be supported a bit longer due to historical nostalgia (one of the few official UT devices) but of course I understand priorities are elsewhere than a 5 years old device.
I was just throwing something out there to see if this could help in reducing the work to make it happen. If not, I'll stay on Android 6.2 until the end of (its) time, I can live with that.