This discussion goes in circles.
Middle class highly educated mothers with DC in good comprehensive schools indeed feel warm inside, and argue for mutually exclusive things. You can't have it both ways.
You can't say that comprehensive system works, and have 40% DC fail, that is leaving school without usable qualifications (5 good GCSEs).
You can't argue that comprehensive cater for all ability, and then add that they are not supposed to work for the bottom 20% because they don't have the ability.
You cannot argue that the bottom 20% need different, vocational courses, and insist that it still is comprehensive education. If you accept separate curriculum for bottom set, you should accept different curriculum for top set too.
You cannot argue there shouldn't be a separation by ability and want top set for your DC.
You cannot argue that some arbitrary X% will never succeed to get 5 GCSEs, and go to guerrilla warfare against ability tests to identify those x%. If you are going to change the curriculum or do something special for the low ability kids, you need to identify them. It is in their interest.
What everybody agrees on is that there should be mobility between the sets/streams. That DC should not be excluded from the academic path at early age without a chance to climb back. This is fair and can be achieved by other means then denying that there is a problem with comprehensives.