I think that part of what drives the uptick in seeking SEND diagnoses in young children is that we have a compulsory school starting age of the term after your child's fifth birthday, yet it's become routine to start sending children to "reception" from four. That's 20% less life length, 20% less development, and these kids are being expected to learn like five year olds, sitting at desks and doing worksheets. Naturally, some of them struggle. I recall once failing to recognise that my bladder was full in reception and wetting myself in class. This would be regarded as an example of a child not being "school-ready". I was four, and whilst there'd have been something amiss if I hadn't been generally toilet-trained, a one-off mishap isn't actually unremarkable at that age.
We are applying the behavioural standards appropriate to five-year-olds to four-year-olds and, instead of saying "some kids aren't suited to full school days at four, and that's OK, perhaps Johnny should stay in half-day nursery for another year and join school in year one", we are treating Johnny like he's a problem, which means that either Johnny's parents are crap at parenting or Johnny is disabled. Guess which of those is more palatable to Johnny's parents? In fact, neither is true and Johnny is within the expected range of behaviours for a four-year-old.
The problem with Johnny staying in half-day nursery for another year has been hinted at by the poster who bemoaned the damage that school snow days cause to the economy: mum needs Johnny in full-day school in order to go back to work. We've collectively started treating schools as daycare instead of the means of educating children.
As long as we keep sending kids to school too young, we will keep seeing parents seeking diagnoses for young children.