Tim is very much still a viable player, but let's get real. He's always been the lynchpin of the Spurs interior offense and defense. While he can still score, his defensive decline, while expected because it happens to all players, has been nonetheless startling. If anyone needs evidence, go back and watch the 2010 playoff series versus the Suns, when they literally pick-n-rolled the Spurs, and particularly Duncan, to death. It was sad watching hiim repeatedly get caught on a switch and then labor to try and get back to the basket - albeit never in time. The quicker Suns guards ran circles around him. Watching the Spurs get swept, it was obvious that Duncan was no longer the dominant defender he once was.
The most recent playoff series versus the Grizzlies, back in April, served as only the latest reminder of how much Duncan has declined. Duncan, with some assistance from an overmatched and overwhelmed McDyess, tried gallantly tried to fend off the young Grizz tandem of Gasol and Randolph, but they were helpless. Both these guys were simply overmatched. Again, it was painful to watch.
This decline is not unusual. As a matter of fact, it's expected if any player plays long enough. No NBA player has ever outrun father time. I just feel bad for him because at 35 years old, he SHOULD NOT be the Spurs best interior defense and best low post player, but that's what he is. It's not fair to him and the FO has done him a great disservice by not providing him with a better frontline support to help offset his diminished play on both ends. For all he's done for the franchise, they owed him that much.