I have no issue with SEND children being educated in mainstream schools. The problem is that it can't be done without a significant increase in the amount of specialist support available including resources and training.
Someone said something about 5 children in a reception class being non verbal. In DD's class there was one boy who was not completely, but largely, non verbal. But mainstream has absolutely been right for him - he was diagnosed with ASD in year 1 or 2 but has come out of his shell, has a small but lovely group of friends and even had a line in the last year group performance! INtereestingly, in the same class was a girl who also has ASD and while on the surface her needs were fewer, that was harder and she has subdsequently moved to a different school where I believe she is doing very well. I noticed at school events right up to year 3 that she basically needed constant 1-2-1 attention which simply isn't practical usually.
But to manage this at a higher level, schools need a lot more resources.
DS has ADHD which has brought some challenges with academics etc. He is slowly but surely catching up, which is great. But at his very large secondary school, they are able to support 20-25 children like him per year group (about 10%) by offering an option for a small number of children to do fewer GCSEs and instead get extra support (with a teacher) during the 9th GCSE lesson time. I think that's quite a lot, but it means if they had more children with additional needs, or children with much more significant additional needs, they'd be very unlikely ot be able to offer a similar level of support.