This is exactly the social cleansing argument, thank you for putting it so succinctly. This is saying that your birth directly determines whether you will be a net contributor, and this is set in stone, so let's try and reduce the scum by deterring them from having more kids.
Look, I'm a net contributor. I'm fed up of scroungers as much as the next person. As I say, I'm no leftie. For example I applaud stricter controls for adults who go on long term sick leave when they don't need to, I think many people should have more resilience and less selfishness. There are things I could say about violent prisoners (taxpayers pay their food and lodging entirely) that would shock mumsnet.
But no child is ever a net contributor, until they grow up. And I'm enough of a leftie to hope that at least some children born to poverty can be lifted out of it, and go on to become helpful scientists, doctors, nurses, engineers, plumbers whatever.
I think it's not just cruel, but bad economic sense to give up on children from birth because they might end up being useless in society. They might not. They might end up being very useful.
You know who else generally isn't a net contributor? Pensioners. And guess what, a plummeting birth rate means we'll have proportionately way more of those.