If 10% leaving causes the school in question to fold, that's £0 received in VAT, and £6,000,000 cost to educate the thousand DC
We have no idea whether schools would fold. But a precipitate drop of 10% could cause significant difficulty to some. And it would be the result of a tipping point, which would not necessarily be 10% (I'm using that only as an example as it was where the previous worked examples cease)
But I agree with the premise of other posters that it's not an outflow in the immediate aftermath that will be the issue. It will be those who decide to withdraw their DC at the next natural break point, or not enter the private system at all. So the full impact will take 3-5 years from introduction to be seen (period between natural break points)
Those parents will be entering the admissions rounds at the normal times, and will have had the time to work out the system, and genuinely move house.
The demographic changes on which the rhetoric of "there will be enough places" does not fall evenly across the country, and the cities most likely to be affected are also those where there just isn't the space to build more classrooms (at all, let alone in the timeframe that matches)
It's going to be bumpy, and in places where there is low demographic decline and a high proportion in private school, it's going to be a difficult time - including for state school parents who will be displaced from the school they might otherwise have expected had distances offered not been affected by higher number of applications and people deliberately moving as close to the school they want as possible. It is quite likely this would start to happen as soon as the next admissions rounds.