I agree, we don't know whether everyone would want to / feel they had to defer. There's some people saying they would / already do (in Scotland) and some people overseas saying they don't - so we just don't know what the take up would be. I also wonder whether in reality people really would - as another poster said, in her area, it was 'pushy parents' wanting kids to start school sooner. If you have a child who is doing well at nursery, happy, confident etc you are quite likely to think 'well they may as well start school, it will be good for them to have something new / a challenge' - a bit like the person who wrote the MN guest post about this, some 'pushy parents' may see it as a good thing to crack on with school. So we can't really say what the uptake will be, that's why I don't think this is a strong argument against the proposal.
'Four is too young to be in school' - yes I know that's a whole separate debate, apologies, it wasn't clear what I was trying to say. There's an argument that parents whose summer born children start school at 5 are having to pay for childcare for that year. But you could just as easily say it's not fair on parents of September-born as they have to pay an extra year of childcare. So should they be allowed to start a year early? No. School isn't intended to be free childcare. It's the start of academic formal learning. If the problem is about lack of childcare, that's a problem that need to be addressed itself, not by making it harder for children to start school later.
BrieAndChilli the point isn't to prevent any child from being disadvantaged, as you say, all children are different and face different challenges. The point of the current proposal is actually more about closing a legal loophole than trying to end all inequality (would that it were so easy...!) - the current legal position is that children aren't requ to start school til the term after their fifth birthday, so an autumn term born or spring term born child could do that and still start in reception- but if a summer born child does it, they'd start in the September of the next academic year, and councils have been making them start in year 1, missing reception completely. So the autumn born and spring born have to right to (1) start school the term after they turn 5 AND (2) start in reception at that time, whereas the summer born child only has right (1), because they can be made to go into year 1 in that first term. That's the legal loophole the proposal is intended to close. The side effect is that they are in the academic year below if they do so. Apologies if you already knew all that though!