Independent sector will not collapse, this policy will not touch the bigger public schools and barely affect the most affluent parents. This is not one step towards getting rid of private schools it is just widening the gap.
It will affect those with two working parents who made other sacrifices or had family help to just about afford fees. Those parents are now more likely to move closer to good state schools, pushing up house prices in catchment areas, pricing out poorer families. Those parents are now likely to pay for tutoring to get into grammar schools (if they live near a grammar school area) or one parent may now not work such long hours (and therefore pay less income tax). Their children will move into state schools already struggling with large classes, poor facilities (and no longer able to use local private school due to cut backs) and lack of teachers. Smaller or cheaper private schools will collapse or merge (happening right now in Surrey) pushing more kids into the state sector.
This is not even mentioning the midyear disruption of children's education (this is not just rumour, I personally know of several students moving schools, or being home schooled from this term) as well as the social and MH implications, let alone the thousands of SEN pupils who will be significantly adversely affected by moving schools.
The policy is utter madness but envy is a powerful emotion.
Oh and the idea that teachers will move from private to state is just laughable.