I'd be surprised if Sally Phillips won't be reading this thread, as she'll obviously want to know how her documentary was received and will probably be very disappointed and also, hurt by some of the very personal comments made.
I've been assuming she was quite likely to read, but I'm somewhat baffled that you think people shouldn't criticise what was in many senses a piece of propaganda, presented by someone who is both comparatively privileged and who has a very personal stake in the topic, simply because the person may be reading. 
I also understand SP's concerns - and a completely valid subject for a documentary would have been an investigation into what will happen to specialist medical knowledge and resources if the population of people with DS dwindles sharply, and/or if social attitudes change to viewing the choice to have a baby with known DS as selfish or ignorant or a drain on resources. I think those are completely valid concerns.
But that's not what this documentary was about. I appreciate that SP was trying to counter doom and glooming representations of DS by introducing the viewer to her own beloved and lovely son, to that very impressive American DS advocate, the gymnast, the Icelandic woman, the Corrie actor - but I think that risks falling into the other prevalent stereotype associated with DS, that they're either cuddly children or exceptional adults.
I wanted SP to put her cards on the table throughout the documentary, rather than making statements about the value of the lives of people with DS. I don't think anyone at all is disputing that here and now in the first world in 2016, unlike the recent past, but I cannot blame anyone for not choosing to be a parent to a child whose severity of physical impairment or LD cannot often be known before birth.
What did SP want? No testing? No terminations for DS? No terminations at all? Compulsory viewing of her documentary before someone makes the decision to terminate or not?
Again, an investigative documentary on what information women with pre-natally diagnosed DS are given by medical professionals/helplines/specialist associations would have been really interesting - but this wasn't it. Or an investigation into what resources exist in order to let people with DS lead independent/fulfilling adult lives, now they're living longer - and what arrangement parents of children/adults with DS have set in place in order to ensure their wellbeing after their parents die?
That was very much on my mind as an older mother doing pre-natal testing - I was 40 when I had my DS - if he had turned out to have DS, I would have had to deal with the fact that he would be likely to outlive his parents, with no siblings and no close family in the UK, and not a huge amount of family money. This would of course have had an impact on whether or not I chose to terminate a pregnancy because of DS, and I don't think it's a selfish position to take, to think ahead to the future of a child beyond your own lifetime.