You are both poorly informed and illogical in your thinking.
My DD was privately educated in sixth form, having achieved all grade 7-9 at GCSE in the state sector. 85% of pupils at her state school achieved grade 4 or higher in both maths and English. She went to a private sixth form because a. a careers adviser suggested she apply for a music scholarship b. she wanted to apply c. we could afford it.
When making the decision that she would move, the 15% of pupils failing to achieve grade 4 at maths and English were not considered by us. It was irrelevant. The reasons parents are prepared to pay school fees for sixth form is that the expense is contained to two years and results are better (private c.49% A and A star, state c.25% A and A*).
My DD achieved A star, A star A (arts), Her then boyfriend A star x 3 (sciences). Both went to Oxbridge. On the basis of this dangerous example of one, I could say I wasted my money. But then, I'm not stupid enough to think grades can be bought, and I understand that As in humanities subjects are harder to achieve than in the sciences.
Around half of private schools are academically selective. About 5% of state schools are (grammar). GCSE results in the state sector are therefore going to be higher than in the private sector.
No reasonable person would write off the state sector as a failure. It fails some pupils. Some schools seriously underperform. Those are the two issues we need to address ie build on and improve what we have.
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