Typical cliche/vanilla take. Sure, center is not as top heavy as it was in the 90's, but only the Celtics and Mavs clearly lack a current legit starter or someone projected to become one.
Valanciunas has his limitations, but he's one of the better low post scorers/two way rebounders in the league. Forget the money and focus on percentage of cap. Once it explodes past $100M next season and his extension kicks in, they'll be paying him less than 15% of the cap.
The concern with West playing center isn't so much post defense as it is defensive rebounding/rim protection. Keep in mind, Duncan is only going to average roughly 28 mpg, which leaves plenty of minutes where West (and Aldridge) will have to defend starters, too.