Ime it doesn't seem to work well for either parent tbh.
Dh just finished paying maintenance for his youngest who is now 18. He began paying £200 pm, his divorce agreement gave two thirds of the sale of the house to his ex and dc. He earned a good deal less than her and I think it's fair to say he really struggled financially - house in a shitty area, barely able to afford days out when his dc stayed, poor relation in terms of the presents he was able to afford for them etc.
But as a working mother, I expect that his contribution (which he increased in line with his wages over the years) didn't even cover 50% of their childcare bills. I do wonder how RPs are able to continue to work after divorce, given childcare costs, if their ex is not a high earner.
I also think that the new partners in the relationship bear a disproportionate responsibility for the stepchildren - on both sides - precisely because child maintenance doesn't work well. I am under no illusions that dh's ex's second husband will have paid considerably more for his stepchildren than dh has over the years, just as since my earnings have overtaken dh's I have been the one to provide for holidays, birthday presents, days out etc. We now have two children together. We did discuss whether it would be fair or feasible to recalculate the maintenance, but we're in a totally different place to when we met - finally financially secure. The difference it would have made to us financially was minimal and we decided the damage it would potentially do to relationships just wasn't worth it. If we weren't as comfortable financially then it would have felt very unfair on our subsequent children too. But then we wouldn't have decided to have dc together of we couldn't afford them.
Dh's ex and her second husband have, generally, been much better off than we have until very recently. But that was the result of their combined earnings.
Child maintenance often isn't fair, on anyone, including subsequent children. But I fail to see how it could be changed to make it fairer.
Incidentally, I wish dh's ex had the same moral responsibility as dh has had the financial responsibility in regards to contact throughout the relationship. We're on good terms now they're older, but it hasn't always been plain sailing (never allowed Christmas day with them, got the dregs of the holiday dates, had to fight every time we wanted to take them to visit his family or keep them longer than his allocated time at weekends etc).