There is a demographic issue yes- primary pupil numbers have been falling for several years now, but secondary numbers are not due to peak until 2027 as the bulge years move through.
It seems obvious that there are some rocky years coming in secondary education when an influx of privately educated kids come into state schools, and more well off kids now stay on in state schools, and the less well off clever kids are no longer able to go to private schools on a bursary or scholarship.
That situation is going to adversely impact particularly the kids who have the least resources among the kids in state schools. Why wouldn’t it? Private schools have been a sticking plaster over the lack of investment into state schooling for a long time and that plaster is now being ripped off with no replacement, basically.
For primary and secondary schools, parents of now ex-private kids, or kids whose parents won’t now send them private; will instead buy into the naice state school catchments, They fill up those places, meaning fewer good state school places for lower income kids.
A lot of those parents will spend on tutoring, creating a further divide within state schools. That then gets higher grades at exams and those grades then pushes up the academic entry requirements for the best state secondaries and sixth forms. That grade inflation through private tutoring in state schools then creates less (or no) choice of schools for then academically middle-achieving and low-achieving kids at state schools.
Same goes for kids in state grammar counties areas. Without kids leaving for selective private schools, academic but more moderately achieving state school kids can’t get into the state grammars because the tutored state kids plus the ex-private kids will take up those places.
Academic kids from low income families won’t be able to get bursaries and scholarships for academically selective private schools any more either, because the private schools are now cash-strapped or closing.