I have to admit I've never having heard of an ob giving infertility as a reason to expect a decline in placental function. Did he have any evidence to believe that might be the case?

Infertility can be caused by a myriad of factors, so I can't possibly understand the correlation between it and placental function. It's too sweeping a generalisation! I don't know if you ever had your fertility issue identified, but even then, no, I can't say I've heard of anything that says that one has to do with the other. Perhaps it's unique to your situation? I don't know. But if I were you, and didn't want to be induced, I'd expect the ob to convince me with facts and proof as to why an induction was really necessary.