It's much, much, less likely. Half of women with down syndrome are infertile. If they do get pregnant they usually end with a miscarriage and there is a 50% chance the syndrome won't be passed down.

Homosapiens are a result of chromosomal fusion, so I would approach the claim of "accruing defects" with great skepticism. Nature is still selecting as evidence by inflicting a high miscarriage & infertility rate.

https://www.ndss.org/resources/sexuality/

https://www.merckmanuals.com/home/children-s-health-issues/c...

They generally do not, either due to defects of germ cell formation or just little sexual activity. "As of 2006, three instances of males with Down syndrome fathering children and 26 cases of females having children have been reported." https://www.fertstert.org/article/S0015-0282(06)03067-6/full...

Because most people with Down Syndrome are sterile

I believe most would choose not to take that step because they are well and truly cognisant of the level of responsibility and care required to raise a child with the same, or more extensive, needs as they themselves had. No one would know better than they would, and I have yet to meet a person with Downs that was stupid despite any manifest intellectual deficit.

That said, it is definitely immoral to forcibly prevent another human from having a child. Any debate on morality must focus on the decision the individual themselves must make.