This is a good analysis of the situation I think, I enjoyed reading it. Do what you want but don't compel me to express your belief about it. Clearly a freedom of speech issue.
Here's the root of the issue, and I'm not sure that you quite get it actually:
Is every birth defect now an equally valid way to be?
What do we mean by "valid" here? You don't really mention it, but there has also in the same time as transexual rights movements the disability rights movements. Obviously "rights" is much broader than actual rights, it's also about affecting what speech is okay (AKA political correctness), anti-discrimination laws, popular media and huge public events (e.g. rallies and Special Olympics), etc.
So it might get closer to the point to ask, is having Down Syndrome a valid way to be?
You can probably lump all these critiques in (obviously most by the left) as a rejection of privileging normalcy, or probably better termed "typicalness". And that's why statistically insignificant populations have a disproportionate voice in the discourse.
In the sense that they have a right to exist once born, but not in the sense that we should put taxpayer money into cultivating the condition and legally requiring people to use special terms to describe it.