These threads seem to follow a spiral:
Assertion: High ability kids need appropriate teaching and challenges to thrive as well as a similar cohort to share ideas with, and grammar schools are an effective and cost efficient way to do this.
Counter assertion: No they don't, they can be adequately served by a true comp. And besides, grammar schools are unfair to late bloomers, kids who had a bad day on exam day, and those just below the cutting.
Next crank in the spiral: Actually, comps aren't serving the most able well. (Followed by loads of Annecdote as well as stats, counter stats, lies, damned lies and yet more stats.)
Further spiralling: parents of very able children are just being selfish and precious in regards to their DC.
I feel like these conversations would improve if we could all agree on Kant's categorical imperative, which is that people must always be treated as ends, not means. the middle as well as both ends of the academic spectrum represent human beings with feelings, potential and the same human worth.