2striped because if 3 out of all the 'oodles of primaries' that feed the school go to the grammar, you've lost the top set. By default, that schools GCSE results will now be lower than they were. Plus, if more grammar schools were allowed to open, as suggested, you'd find more going to the grammars, so naturally less going to the comp.
We're saying that the sec moderns are made worse because that's what the stats show. Consistently, children attending sec mods perform worse than if they'd gone to a comp. Yes there might be examples where this is not the case but most of the time it is, and the research shows this.