Mainstreams don't work for many, perhaps most, autistic kids. That's why 75% have diagnosed mental illnesses by the end of secondary.
Bluntly, primary schools are a huge part of the problem. If an autistic kid is able and masks, they're not seen as having "severe" disabilities. The severity is calibrated on how their disability impacts those around them, and not them.
When my son last went into CAMHS, the triage nurse saw he had PTDS on his diagnostic chart and said, "School?" I said yes, and she just nodded matter of factly.
An autistic kid in CAMHS is seen with PTSD and nobody thinks: was this a car crash? Abuse? Domestic violence? Bereavement? No, their minds instantly go to school trauma.
We removed my son at the start of Year 3. And the trauma is still there. Kids removed in KS3 are generally in an even worse state. I can't tell you the stories I hear, week in, week out. It's heartbreaking - and then I read threads like this, where people think if we shoved these square pegs into round holes with better hammers, the damage might be less.
We don't need more autistic kids in mainstreams. We need mainstreams designed to work for autistic kids, instead. Massively cheaper than state specialists or private schools, to make them smaller overall, quieter, curricula tweaked, all staff trained to understand it, smaller classes. It could be done for baseline fees and notional SEN budget, and it would save a fortune, long term. No more expensive independent specialists at more than £100,000 a year plus taxis. And far, far less trauma being picked up by CAMHS and EHCP budgets, too.