Longevity is a factor, but you seem to rely to much on it. Kobe, IMO, never had the impact and the all-around dominance in order to be the best of his own era.
He could only dominate a game through offense, which isn't bad, but it's not enough either in order to put him in the top 8 of all time. there are just a ton of awesome two-way players that were just better than the one-MVP Kobe - more dominant - more impactful.
Yea Shaq and Duncan are to prime examples but honestly I would have to put kobe over atleast shaq because did fall of big time after he left the Heat IMHO that doesnt take anything away from shaq but at the age of 34 Kobe is performing really well I got to give credit where credit is Do but right now this is what my top 15 looks like but who knows maybe in a couple years it might change players do evolve over time and do become better some of them.