This situation is kind of troubling. I remember back after '03 during the summer debating with my friends whether or not Duncan would play the C. Their argument was that the spurs would have a difficult time even finding a solid C to replace David Robinson that Duncan would be forced to move to the C and plus it was Duncan's natural position since he played it for 4 years in college and would have played it in the NBA had he not been drafted by the spurs. Also they pointed out how Dream played PF at the start of his career then had to play C once Sampson left and how Duncan was in a similar situation. The third reason was that a PF was easily replaceable compared to a center and thus the spurs could find a PF easily if they move Duncan to C.
Duncan can dominate at C but still it just feels weird you know since the spurs have won 3 titles with Duncan primarily playing PF with a C playing right beside him. Also the spurs defensive schemes would take a massive overhaul considering the defense was based on the twin tower system.