Frankly it all depends.
People talk about Kripke having a five year plan and he should stick to that. Well the problem is that he hasn't, in fact he has gone way the hell off of that 5 year plan.
Dean was never supposed to go to hell!
Angels were never supposed to be actual characters on the show.
Sam was supposed to get as dark as he got at the end of season three (and how Dean was able to avoid hell).
Special Kids was never supposed to end the way it did.
There are lots and lots of changes Kripke and the writers staff have had to make to their plan from things outside their control (a couple casting issues and a writers strike),to things completely in their control. Things were as the time goes by they see what works and what doesn't, get inspiration from other sources, ect.
But make to mistakes that 5 year plan has long, long been changed and changed very, very dramatically.
I mean look at season 4, what most consider to be it's best season and almost everything in that season was not either part of what was supposed to happen in year 4, or something that wasn't supposed to happen at all.
The only reason (as of this moment) that I would say their absolutely shouldn't be a season 6, is if the plan for these two characters (or just one of them) is that they are supposed to die at the end of this conflict. If that was the plan, then clearly that is something that couldn't happen with a season 6 (well without a major resurrection and by that time both characters will have died twice, already).
Dorian states,
Not that I want it to end, but the story has built to a climax with this Lucifer business. The brothers' relationship has been through the ringer, they've fought progressively worse and worse foes and worse and worse inner demons. You cannot go backwards in terms of dramatic impact. There's a natural beginning, middle, and end.
Well their is a significant problem with that line of thought. And that is the heart of the story isn't Angels versus Lucifer, thats just the plot. The heart of the story is the brothers character arc, and that wouldn't end with that final battle (unless again if they die). Talk to anyone who deals with extreme dramatic events and half of their story is how they deal with those events after they occur.
A season of "normal, hunting might (as I don't know if the writers want to tell that story, would do it any justice, or would be allowed by the CW) give us a much, much better view on seeing how these events did change the brothers, and how they deal with the ramifications of what they did. Be it a bullet in the brain, going insane with grief, trying to get out of that life, embracing that life to atone, ect, ect.
One of the pluses to this type of story telling is that it would allow the writers to more natural pace the stories. Something that hasn't happened this season. Its heard to go from cities being destroyed to a humor episode and not go what the hell. But you can tell scary horror stories and use them to explore the guys feelings, and not have to worry about having this huge epic story (that happens to be on hold for three weeks).