To me, it isn't about whether one child Is any less valuable than another, its about whether the mother is able or willing to care for a disabled child.
On the less extreme end of the spectrum, you could have a disabled child who is able to have and maintain a good quality of life and independent living just with a little support.
On the extreme end, you could have a disabled child that needs round the clock care, requires assistance with all aspects of living, has no likelihood of independent living, the mother's life will turn into one of a carer, or laden with guilt if she can't provide that care and chooses a care home, and she will grow old with the worry of who will care for them when gone.
Down syndrome often is only represented by the individuals who are independent, articulate and happy, while i imagine the majority are like this, I worked with young adults who have down syndrome who were non verbal, incontinent and had to be supervised at all times due to choking risk or aggressive behaviour, their life is full of value, but mostly, it was the mother who provided most of that care.
No mother should be forced into this role. I've worked in disability care in the past, I helped people live as independently as possible, I loved the people I worked with and they bought value to mine and others lives, just like anyone else. But if I was a mother, I cant comprehend how challenging that must be. Perhaps if it is sprung upon me then I'd find the means, but I wouldn't willingly walk into a role of both mother and carer.
To me, prolifers in this context are just saying to women "why are you bothered about potentially having to give up your career, lose your friendships, free time and hobbies? you should be happy to give up the rest of your life to be a carer that's your duty as a woman!"
I am genuinely happy there are women out there willing and able to do this, and I don't doubt for one second that those parents aren't happy with their child, and have zero regrets about them being born. That's great, there are some seriously strong mothers out there, but I don't think I could do it, and it definitely shouldn't be forced upon mother's.