Certainly, my own and my family's experience of grammar schols is that they absolutely do provide social mobility. My uncle arrived in this country as a penniliess refugee, living in a hostel, and became a top Oxford professor via a grammar school.
I agree that grammar schols are probably less meritocratic than when I got into one, from a not stunningly wealthy background, r when my parents-in-law, who came from v poor/uneducated backgrounds, got into theirs, in the heyday of grammar schools in the 50s, but that is not an excuse for ending the grammar system - it's a reason t return to the best parts of the system as it then was - a local grammar school for every child who reached the standard, so that there was no need for the current caching madness to get one of the very few, precious places.
As stated before, we need to ensure the disadvantages of the system as it then was do not also return ie the return of the secndary modern.
But you have still not answred the point, seeker, why you see that as the only possible alternative type of school in a grammar school system. scarlettsmummy's point stands - that for many kids an academic education in the same schools as the v academic children is not actually the best option.
It seems to me that you are the real snobs, by seeing anyone who has practical rather than academic talents as 'failures'. That's not how the world works. In reality, peple with practical skills are very much in demand and likely to be as well or higher paid than academics. Or teachers.
Being god at exams is one skill, but certainly not the only thing schools should be teaching or the only thing that will benefit all children.
I'm v good at passing exams but humbly in awe of people who can do things I can't, like drive, type properly, make things etc etc ad infinitum. I don't look down on them or see them as failures because they are not as good at exams as I am. Why do you? and why must we all be educated in a ne-size-fits-all schhol when in reality, our interests and strengths are completely different?