"It's easy to think that all children live in idyllic homes where they are loved and treasured by their parents - doesn't more abuse happen in the home than at boarding school?"
Yes - I agree, that children are better raised by caring strangers than family when the family is very dysfunctional and abusive.
It's the same with nursery. Babies are better off in nurseries than raised by abusive or neglectful mothers.
But most families are loving and not abusive.
"The independence so often talked about is inappropriate especially when young, and is a sign of the change children have to make in themselves to survive in an institution...."
I was very independent as an 11 year old. Learned to have panic attacks quietly, learned not to cry. I would travel across the world on my own several times a year - no problems. When I got my period I didn't want to tell my mother as I felt it was none of her business. I didn't want her to talk to me about anything intimate. I was mortified by the thought.
I look back now and recognise that for a very long time I existed with a low level of constant fear as the background emotion. I've only been able to put a name on the feeling as an adult. As a child I accepted it as normal. My parents certainly had no idea that I wasn't thriving emotionally.
I feel I've been lucky as an adult to be generally well in an emotional sense, but that's because I've made a positive effort to stay close to my parents and siblings over the past 30 years since leaving school, and have married into a warm, loving and demonstrative family. I talk to and see my siblings (who are 51 and 46) every week and talk to my mother every day. I would be very reluctant to live in such a way that I rarely saw my family now.
I think my older sister was profoundly damaged by the experience of boarding school. She's a recovered alcoholic and has never managed to have a family of her own - she has very low self-worth despite being a beautiful, clever and hard-working person. Her low self-worth led her into a severely abusive relationship, which thank goodness she's out of now.
Given that most well supported children can and do thrive in day schools - even, state schools, I really can't see the rationale for sending a child of primary age away.