I understand where you're coming from @canileavenowplease. But child maintenance isn't there to redistribute income - it's not a tax. It's there to ensure the children are properly provided for. And requiring a higher earning parent to pay maintenance in 50/50 cases is equally open to abuse, because it means that all the burden of costs is likely to fall on that parent.
I'll give you an example. I have a 50/50 arrangement. I'm not legally required to pay any maintenance. I earn a lot more than my ex - not least because she chooses to work part time (it's not a necessity - both our daughters are at secondary school), but also because she has never had the drive to pursue a high earning career. That's her choice.
I choose to pay her maintenance, even though I don't have to. It's not vast - a couple of hundred a month. That's because I don't want to see my kids going without - even if that's a consequence of her choice not to maximise her earning potential. But I also end up paying all the big costs - she opts out of expenses such as uniform, school trips etc. I pay for all those costs, and she can opt out because she knows that I love my kids and will always pick up the tab. She rarely even gives them money for their school lunches when they're with her - I end up giving them enough when they're with me, for them to cover the whole week. Recently, I've even had the girls contact me in her days because they are going out with their friends, and she hasn't given them any money.
She never does anything with them - they spend their days at her house sitting in thir bedroom, warching YouTube. When they're with me, we are constantly out and about, having adventures, and doing things. That's where all my money goes - holidays, days out....memories. She does none of that.
The girls have come to me several times to complain that they needed clothing or somethig else for her house, and she told them she couldn't afford it, before promptly buying yet another pair of shoes (their words, not mine). So they come to me for it.
In practice, then, she is meeting hardly any expenses associated with being a parent. And she gets away with it.
If your proposal were enacted, I'd be legally required to pay her lots more money, purely because I've chosen to work hard and pursue a successful career (one where I've already compromised on any prospect of promotion since the split, to ensure I can accommodate 50/50). And...because I would still have them 50%, I'd still end up dealing with everything I'm already dealing with. Because she can get away with it.
This is the problem - both men and women take advantage. Your proposal that I should still pay because I earn more simply ensures that my ex has greater potential to asset strip me, without playing her part in supporting the kids. It replaces one form of abuse of the system with another. And the only people who lose are the kids.
Personally, I think the answer lies in social change. I'm damning about those fathers who don't step up. I'm equally damning about those mothers who trwat theirnkids as a meal ticket, and don't believe they have any personal financial responsibility, because it should all fall to the father. Societal attitudes need to change so that parents of either gender who don't step up are shunned.