I think new partner incomes should be largely irrelevant. I mostly think this because it encourages the abdication of responsibility by the actual parents but also IMO it impacts women negatively given that 84% of single parents are women (ONS, 2022).
Any time you adjust maintenance paid by the NRP (usually a male) because the RP (usually a female) has a new partner you are essentially classing the RP as a dependent person, or less charitably a chattel. That has a nasty tendency of leaving women and children vulnerable. Even in a gender-neutral situation it has the effect of transferring daily costs responsibility from one of the actual parents to the new partner. Why should someone else pick up the tab for the actual parents choice to parent separately? In an ideal world a loving step-parent will do this willingly but it encourages at least one parent to not think of the child as their problem any more.
Any time you adjust the maintenance paid by a NRP because they themselves have a new partner, you are usually devaluing the first family (if contributions go down). What message does that send about responsibility for children you've produced? That their value is only worth full price when the parents are together?
Children should remain the full responsibility of both their parents until they are 18. The costs (financial, practical and emotional) should be 50/50 throughout that time. The cost of childcare should be included and there should be no escape.
Non-resident parents should not have the option of voluntarily becoming unemployed to avoid paying anything. If that's a choice they want to make to suit a second family, then the other parent in the new family has a choice to make about whether or not they are willing to pay their partner's maintenance for them. Sometimes it will be worth it, sometimes not. Either way, responsibility to children already present doesn't change because they've become inconvenient for the currently preferred lifestyle. A resident parent can't easily go off and travel the world because they've got dependent DC. It's no different. Children change things for at least 18 years after having them! This is why people should not move in together when one or more of them have DC already unless and until they've been together long enough to have an adult conversation about what that really means. Love does not conquer all!
For every tale on here of NRPs paying through the nose while the other parent lives it up on their money, there are many many more lone parents getting zero or pittance maintenance.
If more parents were actually held to this, I suspect we'd see much more effort made at effective contraception, fewer separations in the first place, and fewer dysfunctional second families (the current social and financial set-up in the UK encourages financial entanglement of blended families at far too early a stage).
No one has a crystal ball and circumstances can change, but too many parents (usually but not exclusively fathers) feel able to procreate but abdicate financial responsibility once their interest in the other parent has waned.