The crown is not passed on hereditarily but rather through imperial elections. The Imperial Patriciate is an oligarchy comprised of the empire’s most powerful and noble patricians who elect the new monarch upon the death or abdication of the previous monarch. The oldest child of the monarch is usually elected to their parent’s position, though this needn’t always be the case.
Should a monarch become tyrannical or threaten the established order than it is down to the imperial electors to oust the monarch and replace them. This has yet to happen, but the power of the electors to remove a monarch helps to dissuade any tyranny that may otherwise come to pass.