I'm not sure I'm that interested in them finishing off the alien invasion arc. By the end of the series, the mythology was such a tangled, incoherent mess that it made . No wonder the last film sidestepped the whole issue with a badly misjudged monster of the week episode. Besides, the series was at its best when it was being playful and light. What's often understated about the show is just how brilliantly deadpan Duchovny played Mulder. Bad Blood, Small Potatoes and Quagmire are some of my favourite episodes. Brilliant writing and the interplay and chemistry between Duchovny and Anderson elevated the already super sharp dialogue. Supernatural owes a large debt to that aspect of The X-Files. It basically laid the foundations of how to do comedy episodes within a serious mythology show.
I think, from series 4 onwards, the writing, directing and execution have aged incredibly well. Season 1-3 are dated, in large parts, but later seasons bridged the gap between 80s/90s cheesy, disposable TV to the serious melodrama of serials such as Fringe.