Duncan will start to decline. His offense will be fine. But he is the anchor of that defense, take him away and they are a very average defensive team, Bowen is great and all, but he is even more likely to decline in the next year or 2 and outside of him, they dont have an above average defender, Manu makes plays but isnt a good defender. Hakeem, Shaq, Ewing and co all started declining on the defensive end when they hit their 30's, offensively he will be fine, but the Spurs are a very good defensive team because of Duncan and when he starts slowing thats the first thing that will go. See Webber, Shaq and Malone for recent examples(Malone got it back to a good level through hardwork but the defense went first). Duncan will still be the best player for at least 2 more years(barring injury) but in 2 years Howard should be there or he will have not reached the level he should, and Duncan's defensive skills(easily his most underrated skills) will start declining because they are more based on athletism.
As for the rest, Manu is already at that age where he should be declining, he has less wear and tear but he plays a painful style.
Bowen is already past the age where he should decline and is starting to become a bit of a modern Malone style ironman.
Horry, Finley, Barry are basically done.
Nazr has no guarentee of coming back.
Rasho is a BWS.
Tony Parker....guy is only 23.....All-Star.....averaged about 21ppg in the playoffs.....he'll be fine....
Also shot them out of games, was the biggest reason they lost(outside of Dirk anyway), rode Duncan to that "All-Star" status and is just not a very good player. One of the most overrated guys going around.
Udrih is all that is left and he is solid, but he isnt keeping them afloat after Duncan and he isnt likely to make up for any decline in a couple of years.
Spurs should be good for a couple more years, they have an excellent coach, the best player in the league and some solid role players another of their biggest problems is that they have basically spent the last few drafts getting backups to Duncan instead of guys who can contribute as starters.