I agree with the OP however Duncan unfortunately hurt his ankle towards the end of the regular season and tweaked it again midway in the first round. Dealing with both an injury and an undersized roster made him look worse than he was....but then again part of being old is that it gets harder to finish a season healthy
However Duncan set the tone early in the series with 10 blocks in the first 3 games. He was also grabbing 11+ boards per game and was shooting over 50%. Duncan was also throwing down flashback dunks that we haven't seen from him all season before he re-injured his ankle.
But after he tweaked his ankle he didn't seem as active. Also he was defending Marc Gasol who is not only nearly a decade younger but also weighs at least 20 pounds more while possessing a wide variety of skills of his own. Duncan also had to help out a lot and since no one else has the size to defend Gasol it opened up the court for the Grizz.
With Dice retiring next season and the Spurs lack of size and athleticism on display in the first round, hopefully Duncan and Splitter will form their own twin towers. But that means the Spurs need to find a better answer for the back-ups than Blair and Bonner. An athletic shotblocker would be nice.
It also doesn't help that the Spurs have a lot of undersized players even outside of their frontcourt especially compared to the Grizzlies and OKC. Spurs never had a real back-up SF, didn't have an athletic shotblocking big, etc.