If one person has earnt 100% of the money enabling the other parent to stay at home they should not be punished for this at separation.
You're assuming that all SAHPs want to stay at home. In many cases this is not true. Many have no choice because childcare is so expensive.
Also, without the person staying at home, the wage-earner wouldn't be able to go out and earn 100% of the money unless they paid for professional childcare. Therefore, part of their earnings should be considered as belonging to the SAHP who carries out that role.
It's not a chicken and egg situation. Usually the SAHP was working before the arrival of children and gives up that role once the DC arrive.
Perhaps it would be fairer for both parents to switch to part-time and share childcare responsibilities, but what usually happens is that it is the man who then pursues his career while the woman stays at home to raise children (or works part-time and thus hinders her career advancement). This is also the reason why 92% of lone parents are women - if children are spending most of their time with their mother before separation, it is obvious that they are going to stay with the mother when the parents split.
So, on separation, the mother finds herself no further on in her career or job experience than she was before children, except now she's 5 or 7 (or whatever) years older with a yawning chasm on her CV. Meanwhile father's career has taken off because he has been able to exclusively concentrate on it while his partner has taken care of all the domestic stuff.
Regardless of who has done what in the past, the point is that no child alive has been born to anything other than TWO parents. Therefore TWO people should JOINTLY share responsibility for that child. In most cases, that will involve the NRP paying maintenance.
In an ideal world, more NRPs would fulfil that responsibility in terms of actual hands-on care, but that is only fair on the child (who is struggling to adjust to life post-separation anyway) if the NRP was doing so BEFORE the split.