Kakashi's job isn't done yet. He has to continue to lead the younger generation.
I mean, look at what's happening: 1 person is singlehandedly destroying Konoha, and he's easily incapacitated Konoha's current #2 ninja.
If there's nothing left Kakashi can show the next generation, then that's pretty sad!
Plus, Kakashi's too young to die. Other than Obito, the 4th, and maybe some bad guys, no ninja have really died as young as kakashi is.
He's too young to die! :p
Plus, I want to see him as an old man, re-reading Jiraiya's books for the upteenth time in a row :p
And we still have to hear about how he got MS.