Jinny, I suspect we dont disagree, and I am aware of children who have been "managed out" of various schools. (And indeed of one child who was managed out of one school at 11, and then achieved a place at a very competitive sixth form at 16.) However being in place already is an advantage and you will go through if you are good enough. Lots of external candidates who are "good enough" wont get places, because there are not enough.
The one thing I would watch though is with the very academic schools. When DS was taking 13+ pretests at 11, SPS essentially told us that if he was offered a place he would be in the bottom third of the cohort. We thought about it and given he was a studious boy, not particuarly competitive, who enjoyed education and having similar friends, some of whom were very clever indeed, we did not see this as a problem. It might have been for DD, who we felt needed a school where either kudos was given non-academic activities or where she would sit within the top third.
Another thing to think of is whether a child is an all-rounder or one-sided. A very kind friend, who was involved in admissions at one of these schools and who counselled me when the HM was advising that our DD should not even try for selective secondaries, put it this way. Could I see DD going to Oxbridge or a top RG University? If I could, we should ignore SPGS as they would be looking for girls with everything. However the rest would be choosing between girls who were either good all-rounders but not good enough for SPGS, or girls who would come into their own at sixth form. The latter group, were often a more interesting prospect, though secondary schools needed to be confident that they would not struggle too much with either English or maths to the extent it was affecting other subjects.
Also interestingly the two schools my friend suggested were the same as the schools my daughter felt most comfortable with. And indeed were the schools who offered her a place. And no, though she applied, she did not get a place at the school my friend worked at - she would have been OK there, but it really was not the right school for her.