The thing is, I don't think there ever was a 'golden age' of grammars when the bright children of the working classes were somehow magically recognised.
There's a great passage in one of JL Carr's novels (he was a primary headteacher in his day job) where it describes the grammar school interviews - this would be 50s/60s - one of the questions the children are asked is "what does your father do", the boys are also asked questions about cricket. Strangely enough, children from the 'right' side of town get in, those from the estate don't.
Similarly, my mother (who actually is pretty positive about grammar schools) describes her school where each year had in her words a token Jewish girl, a token girl from a bad area, and sometimes a token black girl (she was the one from the bad area).
The massive difference in the 50s/60s/70s and the thing that is always ignored is that there was a massive expansion of the middle classes. So the fabled 'social mobility' that people talk about didn't actually include many people going down - only people going up the social strata. And at the same time there was a substantial reduction in income inequality between social classes, and an increase in the proportion of national income going to labour (as opposed to capital).
In that situation, of course there isn't the same pressure to get your dc into the 'best' school - you could be pretty certain that they would get a job when they left school, and also have a good chance that it would be a better job than you had yourself.
And even if they didn't - well, everyone was getting a better income year on year, there was a good social safety net, and overall you expected them to have a better quality of life regardless.