That sounds like a great appt!!! Wonderful to hear of a Dr who is willing to work with
their patient, instead of just bossing them around and controlling everything.
I think there is more to the 20wk u/s than just deciding about abortion - as you mentioned, finding out placenta placement could be very important come labor time, if you had full previa, labor could end up really bad for both you and baby, kwim?
And I think it is important to see how baby is growing and developing. Like you, we would never abort if something was wrong with baby (even if it was incompatible with life), but I would want to know if something was wrong so that we could prepare ourselves/things for the best experience possible.
For us, with Elliana, we (possibly stupidly) were hopeful and expecting her to live until birth, but we were all ready making plans for spending whatever time we had with her. Or if baby had something wrong that would require surgery, or even just something going on that may require some testing or observation or therapy, we would want to have time to get things all lined up, as much as possible, before baby was born.