I have a daughter with Down's, so I probably know more about it that most.
> You will never surpass the cognitive capacity of a young child
This is just completely untrue.
> you will have impaired vision and hearing
There is certainly a higher incidence of vision and hearing problems in people with Down's, but it's far from guaranteed. Further more, glasses and hearing aids are a thing.
> if you ever manage to speak at all
I'm struggling to take you seriously now - this is utterly absurd.
> You will have a myriad other chronic and painful health problems
There is a higher incidence of some physical and mental illnesses in people with Down's, but again, it's far from guaranteed, and again need not have a large impact. For example, a lot of those born with Down's have a small hole in their heart, but nowadays it's fairly easily rectified. Not sure what "painful" health problems you could be referring to.