I think it's a great idea, as long as it's left to parental judgement rather than forced.
Before I became a teacher, I'd never even heard of there being a disadvantage for summer born babies. But over 8 years of teaching, I can't deny that even well into KS2 there is a very strong correlation between month of birth and academic attainment. In my year 5 class there is not a single summer born child in the top 2 ability groups for either Literacy or Maths (out of 5 groups). This trend continues all the way through their academic life as far as university, with obvious likely impacts on their chosen careers and earning potential. The significance is more notable with boys than girls, generally speaking.
So I think it's good that an attempt is being made to attempt to improve the life chances of the children born from April-August and hopefully help them catch up with their peers. Yes, of course there will be some issues, especially with school places etc but something should at least be tried despite possible obstacles.
Yes, of course there will always be exceptions, and I've come across some high achieving individual children born in July and August over the years. But they tend to be exceptions to the rule. It would obviously be wrong to hold those school-ready children back, so they should of course start earlier as per the existing system. It should be decided on a case by case judgement as to how school ready each child is.