Sure, in that instance we’d have King Archie the First — but he’s mixed race. The point being made is that a public institution, particularly one funded by the taxpayer — shouldn’t be able to discriminate on the grounds of race. Genetic succession does discriminate on the grounds of race because mixed race is as good as it’s ever going to get.
It isn’t just the British monarchy though, it’s every public funded or position of power that is chosen by genetic succession. The Kim family in North Korea, the Japanese emperor, African kings — all have the same fundamentally discriminatory on skin colour flaw in their structure.
If you’re fighting for genuine equity across the world, you cannot have a head of state chosen by genetic succession.