I work in a prep school that has boarders.
Most of them are flexi boarders (1-2 nights a week) and I think that is fine - they just view it as a regular sleepover with their friends and have a lot of fun. They're also obviously all local so if they don't like it they can go home.
Of those that are weekly or full boarders, very few are from 'normal', stable, loving, local families. Most of the children are 'fine' but I agree that it's a second best experience for them. I don't think it's as bad as 'surviving not thriving' for the majority - they are happy and get a lot out of boarding - but it's not a family life and they would get more out of being in an ideal home. But the majority of them don't have that ideal home as an option so it's a good thing that boarding is there for them.
We have 11-13 year old weekly boarders from perfectly happy, stable families. Normally ones who have been at the school as day children for years and ask if they can board for their last or two. But the rest, especially the under 11s (who are very small in number) are: military families, parents whose work places them abroad, international students, parent/s who are physically or mentally very unwell, families where a parent or sibling has died, families in crisis or break up of some sort, children placed in the school by a charity, children who have a severely disabled sibling or children with parents who just don't seem to want their lives to centre around their children.
I think the demographic is very different in Senior schools. Most of our Year 8 day children are going to boarding school for Year 9 and I think that's a very different argument. They are going because they've picked a particular school for the environment, education or particular activity if offers, have decided it's something they think they will enjoy and are old enough to thrive away from home (obviously not permanently - senior schools are on holiday for not far 50% of the year if the child is a weekly boarder.) For the added opportunities they offer and the commuting issue many would have to a day school of similar quality, I don't think it makes a parent remotely cruel or uncaring to use boarding school. We have some children who start as day pupils at a famous public school not that far from us. Most become boarders after a term or so because they find they aren't getting home till 10pm and it's not sustainable or much of a family life anyway. Most of these children are from very happy, loving families that give them a great start in life. I don't think they are to be pitied at all.
So, I think YABU for 13+ year olds, YANBU for under 11s and 11-13 year olds it is very child and family dependent.