I suspect that the policy is popular with the 90% of voters in the UK who earn less than £80k and that politicians are not very concerned with the ambitions of the rest of us (frustrating as that is when paying London rent).
The maths for one child for me works out at ~£10k childcare bill without the free childcare vs ~£4000 with free childcare + tax free childcare (for three days a week). Even that doesn't sound too bad but it's the combination of that, the loss of personal allowance and the fact I still have a student loan that means the actual number of how much I have to earn to break even on losing it is something like £30k more than the actual cut off.