Three points:
(1) Why has no one introduced the topic of defense? Duncan is one of the greatest defensive big men of all time, and still one of the best defensive big men in the league, even if his scoring numbers are down and his rebounding statistics have dipped slightly this year. If "defense wins championships," how can we ignore the defensive contribution we would lose by trading for LeBron?
(2) I can't agree enough with PM5K: championship teams without great big men on the inside are few and far between. The Bulls are one of the very few exceptions. Hence, the odds are against us doing better in the postseason without someone like Duncan, even if we'd gain a more prolific scorer and assist-maker in LeBron.
(3) Perhaps no one wants to hear this, and perhaps it's a different kind of consideration, but here goes. In keeping Duncan until retirement, ev
en when he eventually does decline in ability, we move a step closer to establishing the Spurs as a franchise with a lore, a worthy history, in the way that the Celtics and Lakers have established. We have the opportunity to do this as a small market team, and I for one am in favor of it. If Duncan retires as a Spur, it will be like Bird and Russell retiring as Celtics, for example. In my opinion, there's something about one of the all-time greats spending his entire championship- and MVP-filled career with a single team that just imparts more value and worth to the franchise when you look back at its history. There's also a less selfish side to this coin: Duncan has brought championships to SA, where no one else ever has, ushering in a golden era for Spurs' fans the likes of which most sports fans never experience with respect to their teams. One could thus make the case that we owe him the opportunity to decide to retire a Spur if he wishes.