*The trouble with that is - how do you define catchment area?
Maybe when their Year 5 child got accepted years ago, they lived 700m from the school but that would now be far too far away for a non-sibling place. The family haven't moved but they are well outside current 'catchment' for their youngest child.
And that rule also disproportionately disadvantages children of parents who rent and who can be forced to move on next to no notice. There isn't always the option of finding another house to rent very nearby.*
I see your points tiggytape and definitely have more sympathy with those who were in a catchment that has got smaller.
Where I used to live in London I knew of 2 families who owned a house near ish by but has deliberately rented within the catchment of what would have been our local school until we moved, they were open about what they were doing. It is this sort of playing the system I disagree with and would have been really annoyed if one of those families got a place over my dc. Luckily we are not there now.
I take your point about disadvantaging families that rent but the school would have to make a judgement on their local housing stock, in London there's usually lots of rental availablity.
There's no way of making it fair for everyone I guess when demand outstrips supply.