* Koseki as a data structure: It seems to be an event-log from which you can get an official certificate (extract) relfecting the current state. This is better than a systems where you work with loose records of the events themselves. Other countries should do the same thing.
* Koseki as a rigid record of official categories: Soft cultural issues are important here. Koseki happens to force people to change their name upon marriage, but makes official gender-change easy. The Japanese shrug at both aspects.
But English-speaking polities would not shrug, because they have an expectation that legal categories reflect these personal preferences [1]. This is a cultural thing that no data structure will fix.
* Koseki as an alternative to the courts: As long as people will get upset about the categories, and as long as people are allowed to have their legal say, these things will end up in court.
[1] For example, in the UK a "civil partnership" was a marriage-by-another-name arrangment for gay couples (before full marriage was extended to them in 2013). This resulted in complaints from heterosexual couples who wanted to get civil partnerships too.