I didn't go to a grammar (they don't exist in Scotland) but my DH and I both managed to get postgrad level qualifications (PGCE for me, Physics PhD for him). The comprehensive system is brilliant when it works well.
I also taught in a comp with the whole gamut of backgrounds, TAG, SENCo, you name it but every child was (I believe) supported to be the best version of themselves. Some went to Cambridge, some went on to apprenticeships, most to Russell Group unis. Teaching 33 to a class wasn't easy though and more resources did have to be put to those with additional needs, as is proper.
When we were in London, my DS was in the local (outstanding) comp and ended up self-harming due to the 1/2 termly assessment pressure to keep in the top or second top sets (this was in Y8 & 9). So that comp didn't work well for him and we moved out of the city.
Now we are in Kent, he attends a grammar school and feels less pressure, more supported and better taught than in his London school. There are other grammars here that have high % CAHMS referrals due to stress and pressure but not the one he attends. There are students with autism, Irlen, dyslexia, and mental health issues. My DS's friends come from a wide range of backgrounds, from city council flats to large country houses. It doesn't matter.
My point is that each one of us is doing what we believe to be best for our children in the area where we live: there is a great difference between schools and each area in the UK is not the same. I don't think it is helpful to berate other people for their choices - each of us live in different areas with different schools and each of our children are different.
Yes, I don't think the grammar system should exist but it does and, where I live, it is the best educational choice for my children based on their character, abilities and aspirations. If they had expressed an interest to go to the local comp, we would have supported them.
Let's please not slander others or dismiss other's choices. I don't believe that's kind or helpful. We're all doing our best!