Grammars won't solve every problem. But the problem of inequality not seen since the gilded age which is being consolidated at present can be addressed through offering a more stretching, academic education to kids who are capable and reasonably supported. Why is this important? For me, I think it is important to have politicians, judges, lawyers, bankers and corporate executives who grew up with parents who had a mortgage to pay, perhaps had to save to buy a new car, postpone a holiday some years etc. That's what life looks like for most middle income families and people with power in our society should have a deep understanding of how regular people live. After WWII there were plenty of opportunities, supported by appropriate education for DC from "average" families to join these ranks. Less now. I feel this is a problem.
The deeply disadvantage are another issue. No less important. But a much more complicated and much more persistent challenge.
I don't think bringing back grammars will help many in this group. But that's no reason not to fight against growing inequality in our society from the other end.
Grammars solve one problem, but not every problem.