But the problem was, those who didn't make the grade, sometimes by a hair's whisker and sometimes because the postcode lottery meant that there were far fewer places in some areas than others, were then effectively written off.
Grammar schools were an excuse to massively underfund the education of the majority. The idea (not saying it came off, mind) was to offer decent education to everybody, not just those deemed 'bright' by a testing system which wasn't fair across the board.
A lot of intelligent children were massively let down by a system which basically dumped them on the scrapheap because they didn't make the grade, therefore were undeserving of a decent education.
A lot of average to below-average children who actually could have achieved more if they had been considered worth the time and effort were left to struggle.
And yes, sadly, this didn't change as much as it should have after the grammar system, and schools continued to ignore the majority in favour of the very bright, with some lip-service paid to those with the severest difficulties. But at least the top set was a tangible goal and you had 5 years to try and get there instead of this narrow little window which, if you missed it, locked you out of access to the rarefied atmosphere of the bright kids for the rest of your school days.