I would rather it was either kept for all, or scrapped for all.
It cannot be applied to household income as there is no way of tracking household income, the tax system does not link households or even (I think) married couples. The cost of making this change would be prohibitive.
And how do we define household income? Is it based on the people who live with the child (this could include Grandparents, siblings, step-parents), is it the combined income of the people with parental responsibility for the child? What about maintenance payments? What if siblings have different fathers? Who goes past the magic marker and who doesn't?