I agree. Both mine had close friends who were towards the bottom of the year group. More likely because they started at 7+ rather than because they were excessively tutored, and I understand it was not a particularly happy place to be. However both got into good Universities where they thrived, with good study skills and enjoying being further up the cohort. Westminster appears to make every effort to reward non academic achievement, and makes very little fuss about academic prizes or achievement. But there is no getting away from it. These schools are very selective.
DC have had a similar experience. At University DS discovered he was really rather a good mathematician (now taking a quantitative PhD at a US top ten department), something he had never realised at school. Looking back we are better able to identify those who probably had bag fulls of tutoring both to get into the school and whilst there. (One boarder friend of DS confessed, when staying with us, that he had spent every Christmas and Easter at a tutorial college, with only Christmas day off...yes he was in a higher maths set than DS!)
Education is a long path. On average they will probably do about five years of post-school education. The important thing is that they enjoy their education, and this happens by being a good fit with the school. There are plenty of good private schools in London and I am not convinced that you get a better "outcome" for a specific child, as in something measurable like Oxbridge entry, by going to Westminster. That said, neither of mine went to Oxbridge, DD because she wanted something different so chose not to apply, yet they really enjoyed their schooling and left well prepared with the independent study skills required at University, and still genuinely appear to enjoy education.
The problem is not unique to these schools. DD is studying medicine and suspects some of her peers have been heavily tutored to gain the grades they needed. Indeed that some may have been directed towards medicine by parents or others, so struggle with the more vocational aspects. For these education remains hard work, and for some, tutoring probably continues.
That said, big name schools will attract a higher proportion of parents seeking big name schools, and if the fit is wrong there may be problems. For the quirky, bright and individual child, or a child who just enjoys school and does not need extra effort to gain a place, Westminster is fabulous.