I recently purchased and watched all 9 seasons. Despite the last feature length episode of season 9 which wrapped up the show it was still left sufficiently open to carry on if they wanted to.
However despite IMHO season 7 being the best 8 and 9 got progressively worse (despite the fact I really liked Doggit and was partial to a bit of Reyes).
But it was very much of it's time and by the end was almost exclusively about Aliens and such which is a little out dated now. Can't see it would ever comeback for the same reason it went down the pan in the first place, the people involved just grew apart.