Bertrand, I like your killer question.
Indeed, I like a lot of the posts on here because there are so many flaws in any selection system within education.
Going back a few pages someone mentioned Finland which doesn't test its kids through school. I don't think USA has an equivalent 11+ system either but what it does have is a fine post compulsory system (post high school). The rules in Illinois (did a study tour there many years ago) state that no-one can be denied entry on the basis of ability. That means if someone turns up who cannot read or write, the Community College must provide education for that person. Entry test is used only to decide which point in the syllabus they join.
'those of higher ability' – I can't help having a giggle whenever I see that phrase, or the similar ones, 'bright', 'clever', and topped by the opposite, 'oh, but he's good with his hands'. It may be a shock to some but our hands move because we have a brain to guide them!
Our kitchen has a door to a hallway and across the hallway a couple of feet away is our downstairs loo. I watched a guy cut soft backed 'lino' in one piece – no joins – to cover that area. Clever, certainly. High ability, oh yes. Or our lovely SiL, bricklayer, jobbing building, dry stone walls, all highly skilled arts. And at Christmas make sure you're in his team for Pictionary.