I get that it makes sense for one partner to take a step back and for the other to be he higher earner.
I'm not so sure that it does. I think there are a lot of advantages to sharing work and domestic life more equally.
Financially, there are often tax advantages to both of you working part-time rather than one of you working full-time and the other very part-time. (So, for us, one of the main reasons why we're looking at both going to 4 days a week rather than one of us doing 3 days and the other 5 is because we both earn about 50k and both of us doing 4 days massively reduces the amount of higher rate tax - as well as getting us child benefit.) As well as tax advantages (and I accept that these are not as strong for everyone as they are for us - though I think that people often don't look at this as closely as they should), there are advantages in terms of pensions - both of our employers give us quite a lot in employer pensions contributions and we would be quite a lot worse off in retirement if one of us dropped our hours more considerably.
For contingency planning as well, we both feel more comfortable continuing with careers - if one of us made redundant or becomes ill, we have the back-up of the other. Similarly, if one of us needed to go away for whatever reason - we both have close family overseas - we would feel more secure knowing that the other is completely fine to take care of the children. I have seen some fathers be lost when trying to take care of children by themselves.
And, then, emotionally - and I appreciate that this isn't the case for everyone - I think we would both enjoy sharing both work and family life, rather than one of us doing one and the other doing the other.
I'm not trying to say that this is for everyone but I don't agree that it's a given that one person needs to take a step back, I think it's possible for both parents to take half a step back and for that to be quite a positive thing.