It's such a difficult situation and although I did defer my summer-born, I also do agree with your post.
Me and my friend had babies within 2 weeks of each other, with hers being born mid-August and mine being born at the end of August, and so they should have been in the same school year.
I decided to defer my son whereas she sent her son to school just after his 4th birthday so her son has just started year 4 whereas mine has just started year 3.
Both of our boys are very similar in character and ability and when they're in a room together it's very clear they are at near identical stages of development both socially, emotionally and academically. They are both on the same book band when it comes to their reading and they are both on the same level of the "Rocket challenge" - which is a tool the school uses to assess an aspect of the child's mathematical ability.
However, the school has no concerns about my son, whereas my friend has frequently been called in to school due to the teacher's concerns about her son's inability to focus and concentrate, and how he isn't able to retain information as well as the other children, and how he's "behind" in maths and his reading compared to most of the other children in his class and in relation to what is generally expected of a child in his school year.
So although our children are the exact same age and at the exact same level (in maths and reading at least), my child is considered to be doing really well whereas her child is being labelled as 'behind', solely because he's being compared to some children who are a year older than him and so the expectations are higher.
It's a very unfair system.
The reasons I decided to defer my son weren't based around academic abilities or expectations etc, but when I see how differently my friend's son is viewed compared to how mine is, it does make me thankful that I deferred him to avoid having these kind of labels or unfair expectations potentially being placed on him.
(As a disclaimer, I am very aware that my story is an anecdote and it does not represent all summer-born experiences etc etc. I know that the huge majority of summer-borns thrive at school (primary and beyond) so my post, or my decision to defer my son, is in no way any kind of judgement towards parents who made a different choice).