I think the key word is suffer.
Yes, lots of people with Down's Syndrome can live perfectly happy lives, can go to school, get a job, have relationships. However, the condition isn't just a matter of appearance and impaired intellectual ability, associated problems include heart defects, some of which may be so severe as to be life threatening or perhaps incompatible with life without lots of risky, painful surgeries. Some people might consider that it would be cruel to continue with a pregnancy if a child would have severe and painful medical problems that cause suffering.
Personally speaking, if I were pregnant, and if a scan showed the unborn child to have Down's Syndrome, I wouldn't automatically opt for a termination, but I would possibly consider it, not on the basis of Down's Syndrome, but taking into consideration other medical complications and complexities and potential suffering and potential need for painful medical interventions in future.