Can't find the stats but this gynae/obs doctor, Dr Jennifer Gunter, in Canada does a great job of addressing these questions with medical knowledge and evidence (see her twitter).
My understanding is that such late abortions are extremely rare, and more likely to be for other major problems such as Edward's Syndrome , another trisomy, in which babies are terribly disabled, only half of those born alive live a week and most of the rest die in the first year.
From a personal point of view, I have extremely complicated pregnancies which leave me with long term health problems, so if faced with carrying a pregnancy to term, which would result in a baby unlikely to survive for long, it would not make sense for me to continue with the pregnancy, for either the baby's sake, or my own health. I would hope to make the decision earlier in the pregnancy, but by the time scans, amnios and consultants have been seen it could be well beyond 24 weeks.
I would not want to answer for someone else. The situation of a woman in a well supported relationship, with good physical and mental health, and plenty of money might be very different to someone who would be on their own, broke, and suffering clinical depression, in terms of whether they could reasonably support caring for a potentially severely disabled child.
In Down's syndrome the range of impact is quite wide, and I've friends and family who have Down's Syndrome to different degrees, from:
(1) needing to be in a care home, many connected health issues such as heart problems as well as needing constant care
(2) able to live in a supported care facility but independence will never be possible, deeply unhappy about her life and prospects, and
(3) actually turned out to have mosaic Down's syndrome, so likely to be able to live with support, and potentially hold down a simple job.