some people who pass the 11+ actively choose to go to the comps instead (those who need support in other areas or who fast-paced grammars may not suit) As well as very high achievers who do not even take the 11+, for a whole variety of reasons - they have a good comp on their doorstep, the super-selective would require a long journey, the parents prefer a faith place, or make an ideological decision, or who may not even know that a super selective exists or is a possibility, or known that aged 10 their child might be a contender.
But Tiggy is right - it would seem that super selectives have little impact on the comps on the area, either in removing a cohort of bright kids, or, crucially, in determining the breadth and depth of curriculum.
Schools called Secondary Moderns don't actually exist any more. In selective areas (e.g Kent) I understand that the non-grammars are called 'high schools', or community schools. Secondary Moderns were a feature when there was a two-tier exam system: O Levels and GCSEs, however, at least high schools now teach the same GCSE curriculum.
I have seen MN parents say that high school students are not offered the option to do more than one MFL. Latin is beloved of many grammars, but may well be more rare in high schools.
So does one day, in the life of a summer born boy, for example, which sends them to one type of school or another, determine outcomes for their future? Well, for a child who is a bit of a late starter, such as your summer born boy, who may well have gone on to be a classicist at Oxford, yes, almost certainly.
On the other hand, for a child who is practical, determined, ingenious and keen to learn vocational skills and get out earning in a skilled trade (the new sort as well as the trad ones), hopefully a Comp, as well as a high school, can offer a good range of BTECS, and have the facilities to do so.
You can read about the old secondary moderns here