I get where you're coming from, I think, OP. We are sort of in the same situation, though we have not had trouble with infertility. In our situation, dd was a planned only. I had her when I was 36. Then, last year, we decided to try again after all. Dd had just been so much fun and she was getting older, and we were in a better financial situation and, and...
So at 42, much to my surprise I conceived within 2 months. Dd had always made it clear that she did NOT want a sibling. She was very articulate about why, too. Again, in our case it was a very casual conversation. It does come up! No, she has never had a "say" in our decision; but living with her day in and day out, having her make comments here and there and knowing she is very, very happy as an only and we aren't exactly pining for another child either, well, it does change the tone of OUR conversations about it a bit, rightly or wrongly.
After I miscarried, I was diagnosed with an ovarian cyst. We were told not to TTC until it resolved. It has now been 9 months and the cyst is "persistent." We still really shouldn't TTC. I am now 43.
If we really, really wanted another baby, I would be pressing harder for a referral to a specialist to deal with my cyst. It has been imaged and tested repeatedly and deemed to be noncancerous. It can be watched for now, unless I want to have a baby. Then it should probably be removed first. If that was the route we wanted to go, I should be demanding an appointment at the infertility clinic and prompt surgery. As it stands, I haven't done that. I think it's all part of the "letting go' process for us and somewhere, that little voice of my dd does whisper in my ear. Is she part of the decision? Not really, not consciously. But knowing she's happy with the status quo does help move us along a little. I think if she were "begging for a sibling" then yes, I might actually be swayed a bit in the other direction.
It might just be that her feelings help us feel like maybe it's okay, just the three of us. Maybe we should just focus on that. But I think those our our feelings, more than dd's.