A year ago, the Celtics also had a young, dynamic PG still in his prime (Rondo/Parker), an aging but still crafty scorer (Pierce/Ginobili) and a old but still effective defensive leader (KG/Duncan). We also had a coach widely considered to be one of the best in the league (Rivers/Popovich). So why couldn't the C's just continue on and supplement the aging stars with effective bench guys with late-round picks/smart free-agent signings? Was it just too difficult a path to take? Obviously the situations aren't exactly the same but they're not entirely different either.
Here are the reasons I can think of for why it didn't/couldn't happen here:
1) Doc doesn't play the kids. We all know it, Doc doesn't play kids because he doesn't trust them. Thus, the bench was never allowed to develop and the old guys played too much, tiring them out when the playoffs came along. The Celtics always said they wanted to play Pierce and Garnett limited minutes but they never had the discipline to truly stick with it.
2) The C's stink at finding international talent. For whatever reason the C's never exploited the foreign market (Dino Radja where art thou?) to their advantage. If you believe the stereotype, foreign players are more team-oriented and have better fundamental skills like passing and shooting.
3) Duncan is obviously aging better than Garnett. Duncan isn't the same player as before, but seeing him on the court he can still move pretty well and his game never relied on athleticism anyway. Garnett does not seem like the same player since he lost his quickness and burst.
4) The Spurs stars all took discounts. The Celtics stars did take slight discounts on their last deals but they were not significant enough to give the team more flexibility. Their attitude seemed to be that they still wanted to make good money on their last deals which would last past their primes. Was Pierce worth 15.3 million this past year? Garnett is not making a crazy amount (12 million/year until 2015 to Duncan's 10 million) but his production is dipping precipitously.
5) Ray Allen wouldn't have left the Spurs (or the situation wouldn't have happened the same way). The Spurs seem to have a great atmosphere where everyone on the team loves each other. This is not limited to on the court, as reports are they enjoy spending time together off the court as well. I think the C's do have a strong organization, but as we all know the team relationships were sometimes a little frosty. Maybe some of the blame falls on Ray here for quitting on the Celtics but I'm just throwing this out there for consideration.
6) Ainge picked the wrong supporting guys. One of the reasons the Celtics couldn't win another title is they didn't get great FA pickups or good late 20s draft gems in recent years. You'd think that after the first title Ainge would have his pick of any mid-tier free agent but it didn't work out that way. Instead, we got the O'Neals (Shaq and Jermaine), Rasheed Wallace, Marquis Daniels, Jason Terry, Stephon Marbury, and Patrick O'Bryant. Meanwhile, picks like Giddens, Melo, JaJuan Johnson, and all the 2nd rounders Danny took failed to click. Back in the day Ainge would turn late first rounders into gold like Tony Allen, Rondo, and Perkins. After the title win he definitely had a dry spell until he took Sullinger.
Well, what do you guys think? Of course there are other reasons (injuries primarily, but those also hit the Spurs in recent seasons) but these seem to be the main ones. Or, it could just be as simple as the Spurs from top to bottom are just better at everything. They're like the Patriots: past their title prime in recent years but they're always in the hunt and they seem to effortlessly change the parts around Brady (Duncan) and continuously win year after year.