As another KCS parent, like Longdaze I don't really recognise this description of KCS either - like so much in life, the picture isn't really black and white.
In addition to academics there were other issues at play regarding the boys weeded out after AS recently. Actually the school seems to me be quite good at supporting those who struggle a bit academically and still lets them take exams (and get e.g. a D...) which rumours suggest some other schools don't allow. Overall I've found pastoral care to be very good.
That said, there was a serious issue with the last academic year's upper sixth which, with the benefit of hindsight, the school handled badly - this caused some staff turnover including the loss of some fantastic individuals which was a great shame. Staff turnover happens however... And things have subsequently settled down again.
The introduction of the new lower school is primarily in order to make the school more accessible to boys from state primaries. Who can argue with that! The lower school will run in parallel with the junior school at least until all current junior school boys have moved through the system and got what they signed up for - beyond that who knows, the school is saying no plans to merge but it's widely accepted they're probably being a bit disingenuous about that...
Very very few extremely wealthy Russians etc. Yes there's the odd one or two, and of course you notice them!, but statistically not very significant.
Sport is a bit of a weakness unfortunately. But it's an academic school and that's where the emphasis lies.
The headmaster isn't on sabbatical at the moment, but next term - he will return at the end of term and will be available over the summer to deal with the fall out from public exams. Not much goes on in the summer term anyway due to all the public exams, and the principal deputy is absolutely outstanding (in some ways she's probably better), so I don't think this is a big deal at all - but fair enough if others do.
Overall it's a forward-looking school which is willing to change and move forward with the times - although I can understand the point of view of wanting the school to stay the same as when you joined, I really like the progressive attitude and feel that most changes are a move in the right direction. E.g. I was delighted that they brought back A levels - very few boys are genuine all-rounders at this age, and many prefer to specialise e.g. in sciences or languages or humanities in a way which A levels allow and IB doesn't. And universities don't like IB for e.g. medicine. The fact that the majority now do A levels suggests that their reintroduction was definitely a good move.
Interestingly, from speaking to other parents, it does seem that boys' - and therefore parents' - experience of the school does vary to quite an extreme degree, depending on the nature of the child. (Perhaps this is the same at all schools..?) Boys who are confident, bright and musical or sporty or whatever, seem to be served extremely well - anecdotally however, less confident, middle-set boys might feel a bit invisible, and there are stories of some very sensitive, quiet boys having periods of being utterly miserable.
There are also stories of boys who were top at their prep or primary getting very depressed to find they are below average in a very bright cohort. Obviously this is not the school's fault, but something to bear in mind - personally I think KCS is an outstanding school, best suited to genuinely bright and confident boys (who may or may not be musical / dramatic / chessy / sporty / whatever, that doesn't matter so much), personally I wouldn't choose it for a hard-working plodder or a very very gentle, sensitive type. Just my opinion!