What is accomplished in that time also matters. The USA in ~250 years has done more to destroy human civilization than any other nation except fr perhaps the British Empire, and the Empire rose to its heights when there was a balance between the crown, commons, and lords, IE a hybrid of Republic and Monarchy.
Fixed that for you.
You're making a big mistake in assuming that all monarchies were unbroken lines from beginning to end. In reality, Monarchs and their governments failed and were overthrown constantly, just to be replaced by new Monarchs. If you look at the history, the vast majority barely lasted a few centuries at best (and more often, less than a generation), before some sort of violent struggle for power occurred.
Monarch is not a monarchy. By that measure, democratic governments are overthrown once every four years. And many monarchies avoided civil wars by having elections.
Kingdom of Croatia for example lasted from 925. to 1918., or some 993 years. During this time, we changed dynasties like shirts: Trpimirović (845. - 1091.), Arpad (1091. - 1095., 1102. - 1307.), Anjou (1301. - 1395.), Luxembourg / Habsburg / Jagellon / Hunyadi rotation fest (1387. - 1526.), and finally Habsburgs (1527. - 1918.). And with maybe two or three exceptions, most of these did not end up in civil wars or armed overthrows.
Theoretically. Unfortunately in practice, it rarely worked out that way; which ultimately resulted in the rise of Communism. Then of course there was the incentive Kings had to loot other nations, which caused the World Wars.
World wars were caused by
governments, not by
monarchs. Some of the loudest calls for war were from France, which was a democracy, and wanted revenge for the defeat in Franco-Prussian war. Monarchs themselves in fact opposed the wars - yes, even Franz Joseph - but between the pressure from the governments and pressure from the populace, they had little choice.
Look, Monarchy is neat in theory, but there's a reason that when Fantasy settings want a "Good Monarchy" they either go with "This particular King/Queen is good" or "LITERAL DIVINE INTERVENTION" to make sure the Monarch is good.
Got any telepathic white horses from God to make sure the Monarch's a good person?
No need. Rather than looking at fantasy, look at history: there were plenty of good monarchs without divine intervention. And in fantasy, divine intervention is not there to ensure that there is a good monarch, but rather to bless / confirm an already good monarch - this is a literaly tradition that is based in Christianity. Look at
Lord of the Rings - Aragorn was a good monarch, and he did get divine blessing, but he wasn't
installed by divine blessing; blessing happened after the fact.
One reason for this is also why monarchs were generally better than people today assume, and why they are arguably better than democratic governments: monarchs were blessed by God. Historians today focus on the rights stemming from this, such as suppression of rebellion: but in reality, there were also duties. Monarch that was annointed by God also had duties laid down upon him by God - and if he did not fulfill them, there was a basis for rebellion against the monarch in question.
Keep in mind that this is a medieval concept, and very different from the "divine right of kings" which appeared in modernity. Nevermind that today we have the "divine right of human rights", "divine right of Karl Marx", "divine right of COVID" and so on justifying literally anything our political class might come up with.