"A parent with a child at boarding school is just as much of a parent as a parent whose child is at any other school or none - they make parental decisions, offer support (both physical, emotional and financial), are the people their children turn to about all sorts of issues, spend more time with their children than anyone else -
They are NOT parenting in the same way as a parent who is living with the child.
They're just not. My relationship with my children is intimate, affectionate and dynamic and it really, really can't be conducted in the same way over a telephone than it can when we are together. I don't just listen to my children from a distance - I see them, watch their facial expressions. We communicate by touch and sight as much as we do by words. The idea that you can conduct a relationship with another human being in the same way over a telephone or by text as you can when you are in the same room with them? Really?
"they just do a very small proportion of that parenting from slightly further away."
If they only see their children at weekends and holidays then their child spends more time with friends and paid professionals than they do with their own parents. If they are full boarders, as I was, then they spend massively more time in the company of other people than they do with any member of their family.
I wouldn't tolerate my DH spending all week or even months at a time away from me if it wasn't utterly essential to keeping body and soul together as a family. I certainly wouldn't want him to do it if he had the option of a job with a shortish commute. I accept that some people feel that state schools are not good enough for their very clever or talented children, and I accept that some people don't have an alternative because they have NO schools within a reasonable commute, and that boarding school really is the only possible option. But I suspect that they are few and far between.
Incidentally have you given any thought to how boarding school disrupts the sibling relationship?
My children play with each other and talk to each other all the time. At home. At school their relationship with each other is shaped more by the structure of the school and their peer relationships than it is by their family bond.