If you are a SAHP affected by the reductions or loss of CB, then frankly, you could start earning to make up the difference. Your partner will be earning at least 50k, in fact 60k if you stand to lose CB completely, so the argument that you can't afford childcare falls a bit flat
Conversely, if you have a couple each earning 45k, they are already paying childcare out of that, so probably have less disposable income than the first family
There are lots of costs to working - childcare and transport being the main ones. A SAHP is highly unlikely to need a second car for instance. I actually took my car off the road while on maternity leave to say a bit of money- and no I didn't live somewhere with great transport links, I just spent a lot of time at home or out walking. Whereas once I went back to work, I was paying nursery and needing a car to drive there and then on to work.
I'm afraid I can't muster huge sympathy for families with a HR tax payer and the luxury of a SAHP.