Here's his own thought on the matter. You can watch this with the autogenerated translations on if you want to get a feel for the real Nairo that never comes through in the English-speaking media (veteranas means cougars in Colombian slang, BTW).
It actually seems like Arkea might go up to WT next year, and they seem to have promised Quintana some climbers. I mean, it's good to stick with a team you already went through some tough times with, so you know exactly where you stand. But try as they might have wanted to they were very little help this Tour, their second best climber is mercurial at best, and they focus on the Tour, instead of the Vuelta and particularly the Giro which would seem to should suit Nairo better.
I think he's a notch below Bernal/Hindley/Carapaz and another one past Pog/Rog/Vinge. But he's a wily racer and he risks it all in a way that prefaced the current generation, and by that I really mean Pogacar, Carapaz and Bernal in terms of attacking intent. I still think if the race breaks his way he could still podium a grand tour. I do think he still dreams of the Tour, though, and that might be why he sticks around.