What might be the case is that your backup was significantly worse than your starter and that would explain why the coach left your starter in for so long even in a blowout.
Well my backup is better in JR, OD, HD, and DR. the skills that are the same are passing and ID. My starter is better in IS, RB and SB.
When I compare my starting PF with my backup PF, the difference between the starting PF and backup PF is much larger (starting PF is better in most of the skills), yet my starting PF got 35 minutes and backup PF got 13 minutes, so it doesn't really go with that you said.