I find it quite strange.
All this talk of advantage and two tier system.
There are lots of amazing private schools and lots of mediocre ones and some that really aren't great.
There are lots of amazing state schools and lots of mediocre ones and some that really aren't great.
No sector has an equal offering, and not all schools will be right for all children.
DH and I were both very academic, high achieving and needed to be in schools that focused on that.
DD is high IQ, SEN and not interested in academics. Neither DH's super-selective private school nor my super-selective grammar would have worked well for her.
We looked at some incredible private schools that were not just catering for the strings-of-9s, but ultimately felt the state comprehensive she goes to had the better offer for her personality and interests. Her comp has a better offer in every way than my grammar and I reckon she's had a nicer childhood there.
Not all parents are chasing the highest grades possible. The idea that you make 'contacts' through private school is laughable (those that look like they did already had those contacts through parents social circles).
Most parents are paying for small class sizes, or extra-curriculars on site that enables them to work longer hours, or live somewhere with a pretty bad state offering. Some are paying (or the government is) for specialist schools whether that's ballet, or music or SEN.