If you are in the position of making a choice, right now, in your own circumstance, and a grammar is the best option amidst failing schools or out-of-reach housing, then who wouldn't take it? Any parent would, me included. If I lived in a fully selective area I would be hoping that my 'high attainer' DC dud as expected on the day and got a grammar place.
That is different to discussing whether in principle, strategically, grammar schools are a 'good idea' and should be introduced in Wales.
There is no doubt that in general grammars work for the young people who get in. But education for a society, a C21 country, needs to be able to get the best of these and all other young people. We need a system that allows children to develop the best of themselves at the right pace at the right stage in their lives. Pick up the maths genius, even as they have only average literacy, recognise the boost of a summer born boy as he matures academically at yr 9, give specialist supports dyslexic children , support children from chaotic homes to reach their potential . Supporting vocational education that matches the modern opportunities, inspiring a curiosity in learning and confidence in their abilities amongst the disaffected .
Ultimately an education which serves every child will benefit the world that the grammar cohort find themselves living and working in.