I agree that it is irresponsible to have children you can't support, and it can be interpreted as cruel to the child. I also think people should be discouraged from having children in general, because I do think overpopulation is a problem.
Yet I still think it's abhorrent to actually expect people not to have children they can't support, because the ethical implications are despicable. The natural conclusion to that line of thinking is that only those who are born to advantageous positions in society should breed. That anyone who isn't born into privilege and chooses to breed or makes a mistake deserves to be, at minimum, judged.
There's an irreconcilable contradiction here in the values and expectations society expresses towards poor people. We encourage those who aren't born with silver spoons in their mouths to be patient, work hard, make responsible choices, etc, and eventually it will pay off. We shame people who are in their late 20's and complain that they still struggle. We call them naive and entitled. But what if someone really wants children? Do we still expect them to wait until they're middle-aged and finally achieved stability to pursue what they really want in life? Woops, too late. Sucks to be them. Guess all that stuff about hard work and patience wasn't really aimed at people who are family oriented, huh? All we really meant was they might be able to party through the later half of their lives. That's apparently all society believes anyone should aspire to.
It also invalidates the more somber lines you hear pushed on the poor sometimes about how those born into poverty shouldn't expect their hard work and patience to pay off for themselves, but they can set their children up to live better lives than they did. I hear this sort of thing a lot to convince the poor to be humble, and nobly work their asses off for the sake of future generations. But combine these two sentiments and what it really means is 'Yeah, you're just fucked. Please continue being a slave for us. But also don't make any more of you."
The practical implications suck, too.
Even for people who manage to reach financial stability at a late-fertile age, there's a direct correlation between a women's age and the likelihood their offspring will have various health problems. You're literally asking people to produce a population that will have more health issues. And while it may be financially easier to raise children later in life, it's harder in many other ways. Have a child at 20 and when that child is a teenager, you can still keep up with them. Not so if you have a kid at 35. A 35 year old also won't handle the sleep deprivation and energy needs of a young child nearly as well, either, and will thus be more prone to neglect or make accidents in caring for them. Yeah, they'll be able to provide better materially for their kid, but that's not all their is to raising a kid. The expectation we're talking about tells the poor that they should risk their child's health and offer them lower quality parenting, in order to provide for them better materially.
Finally, what we're talking about also results in money management being our #1 factor in natural selection. Not only are there many, many other qualities that can be desirable in a person that we should want to be passed on, but getting rich often involves being a fucking asshole, to put it bluntly. The corporate executive demographic has a pretty fucking high concentration of sociopaths. That's not what I want the evolution of our species to be based on, thanks.
If people want children, they should be able to have a couple children, and receive whatever support they need to be able to properly raise their kids. Regardless of whatever their financial circumstances are when they make that decision.
And ultimately, there's still no excuse for poverty in the modern world. It's something that at this point is enforced. The proper response is to fight back against the people that are enforcing it, not to encourage the victims of class war to make such sacrifices.