Doc has stated that real C's are almost nonexistent.
When did he say that? I mean, there are a lot of power forwards who play a lot of center, and at a pretty high level (Aldridge, Noah, Horford...), but there are a pretty good number of legitimate centers out there. Bogut, Bynum, Gasol, Gortat, Howard, Nene, Duncan, Perkins, McGee, Pekovic, Monroe, Cousins, Chandler...
Lotta good centers out there. Also, just because a center can play the 4, doesn't mean they're a 4 playing the 5, anymore than when a 4 plays the 5, it doesn't mean they're a true 5 playing the 4.