"I am saying that there comes an age, currently 16, when people will make their own choices and go their separate ways."
Yes - it's entirely arbitrary and not linked to intelligence and academic potential. It was fifteen, currently sixteen and will be eighteen. Are you suggesting there should be no selection at sixth form either? What if education was made compulsory until 21?
Everyone on this thread, just about, is in favour of selection, via setting. Nobody is advocating a truly comprehensive education where all abilities are educated together.
So IF grammar schools do offer a superior education to its pupils you have to come up with really excellent, watertight arguments to dismantle that and "bring them down" to the level of the rest.
IF they don't, there shouldn't be a problem for you.
And if they DO offer a superior education the answer is not to remove that - a retrograde step, to educate children more badly? - but to make the education of the others just as good, if not better.