> The government is most certainly legitimate if they're able to hold a country together for centuries.

Legitimacy based on force is not legitimacy. It's just tyranny: if you never ask people how to want to govern themselves how can you meet the criteria of legitimacy, like EVER? Holding the biggest guns does not replace agreement and contractual acceptance of how things are run.

> Should "tyranny of the majority", as they'd say, be considered legitimate

It's a hell lot more legitimate since you can actually expect to change the rulers/people in power at some point through popular action. Under royalty/tyranny, you are forced to accept whoever is at the top and you have no recourse until they die.

That's kind of a huge difference.