The tripartite system - of which Grammar schools were one part - met a perceived need of the 1940's.
State funded Grammar schools only existed for a short window between 1948 and the late 70's
During that time lots of meejah commentators got their breaks and so fed a view that Grammars were just wonderful.
In the long run there was no evidence for that view.
Pre 1948 the school leaving age was under 14 as most kids had manual unskilled jobs.
Since then the school leaving age has steadily risen to now 18
with a default of 21 for those wanting careers rather than jobs
(NB I include those doing courses like Sparsholt and Southampton City College and Totton Colege and Eastleigh College in the 21 year olds - A levels are just a small part)
THe sorts of people who think the return to grammars is a good idea are often (in real life) the ones who think points based immigration is a good idea
while forgetting that most immigrants pick vegetables, swab toilets and cut up dead animals for supermarkets.
Grammars are like Hinkley - they deal with the problems of yesteryear, not where we are now.
Comprehensive schooling that allows flexibility between outcomes is by far the best way to go.
If only we could get rid of parental faith selection on kids we'd get even better outcomes.