Succession struggles were like their own whole genre of political strife until the modern era. Multiple parties can have claims of varying legitimacy -- first born might gain some advantage in being near to the previous leader, inheriting the royal rolodex and hopefully some powerful/motivated allies who want to keep the status quo, but it isn't a sure thing.
Sure, now that the top position is entirely symbolic in the vast majority of monarchies, nobody fights over it. But the if the UK Monarch was in any sense "in charge" of anything, we'd surely see the US propping up Harry and Meghan as the true legitimate heirs and we might even let them borrow a couple carriers to "persuade" Parliament of the fact. Or whatever.
Sure, now that the top position is entirely symbolic in the vast majority of monarchies, nobody fights over it. But the if the UK Monarch was in any sense "in charge" of anything, we'd surely see the US propping up Harry and Meghan as the true legitimate heirs and we might even let them borrow a couple carriers to "persuade" Parliament of the fact. Or whatever.