Inclusion of high-needs kids happens to the detriment of all others, however you look at it. Yes, in an ideal world, there would be enough funding and enough resources to deal with them, but there just aren't, and so everyone is failed.
EHCPs do not take into account individual schools' circumstances. They just make a load of requirements that most schools cannot fulfill, therefore making schools fail their legal requirements, pushed onto them despite often numerous conversations that they just cannot provide said support.
Some real-life, extreme examples of EHCPs which affect everyone:
I have one class with so many EHCPs and other needs (all of which require the child to be sat at the front of the room) that I cannot fulfill their legal requirement for seating. So I choose the visually impaired kids over those that might need it for behavioural reasons, because the needs of the former trump those of the latter. But I cannot follow their EHCPs.
I have one physically-disabled child who has to use the lift to get to classes because they are in a wheelchair. Lovely child, no behaviour issues, but in our crumbling building the lift breaks down regularly and if you cannot get a repair person out for 3-4 weeks then all lessons have to be re-roomed for the class. Not an issue in many lessons despite the disruption it causes for all, but I teach Science and need a lab. There have been numerous occasions over that child's educational course where all practicals for the whole class had to be cancelled for this reason. Their right to an education following their normal timetable has meant that 29 other children also missed out on practical work.
I taught one child who regularly threw furniture when agitated. I cannot count the number of times I have had to remove the whole class from the room because they got angry. I had two children with severe anxiety in the room, who were terrified of the child. I also had a child with BESD in the room, and several others with SEND, who made it a sport to agitate said child, partially because they knew it would mean a break from lessons. The parent was desperate for a special school place, but the LEA denied them one. The child was one of the main reasons I left that school, because I felt unsafe with them in the room.
Children are not permanently excluded even if they physically injure staff and students, unless this has happened a few times. One member of staff I work with has a permanent injury to their leg, which they got when a chair was thrown their way. The child had an EHCP, was not permanently excluded and the staff member was expected to still teach the child with all the grace they could muster up. The member of staff ended up not only with a life-changing injury, but also with anxiety and PTSD that requires permanent medication.
Schools are not equipped to deal with this, and the sheer numbers we have getting through the doors that we are expected to cope with are ridiculous. A school that states they cannot meet the child's need does not do so lightly. Some parents still insist, some have no choice. But everyone is, ultimately, failed and the neverending question is whose needs trump whose.