The elephant in the room that hasn't been mentioned as much -- money.
Duncan knows he can play 2-4 more years and make a lot of it.
And given his new personal situation, he may need it.
The rest of it cuts both ways. I get the impression that Duncan is taking refuge in basketball right now --it's a release from all the rest -- and that pure focus shows in his play. On the other hand, if he has the kids, basketball will take time away from them.
I always thought that Duncan could be like Jabbar -- effective at an advanced age because he depended upon length, footwork, finesse and smarts rather than athleticism. Then I thought I was wrong when TD declined at an earlier age than my theory predicted.
Now, I'm back to the original Jabbar model. I think TD plays till the wheels fall off.