2) The most powerful kingdoms in Middle-earth for most of its history were human ones.
Despite the Elves’ glorious hair and superhero-like abilities—as in the Peter Jackson movies—and the Dwarves’ reputation as the hardiest warriors, the greatest kingdoms ever created in Middle-earth were those of Men. In the Second Age the Atlantis-like human kingdom of Númenor was the most powerful in the world in terms of both naval and land power. The Númenóreans were 7-feet tall, built ships that plied all the seas of Middle-earth, wielded steel bows and so cowed the armies of Sauron that at one point they ran away, leading to the evil demi-god being taken captive. Unfortunately for Númenor, its arrogant King Ar-Pharazôn was convinced by Sauron to invade the far western land of Valinor, where the immortal Elves were allowed to sail in order to dwell with the angelic Valar. This resulted in Númenor’s complete and total destruction. But refugees from there, led by Elendil, escaped to Middle-earth and organized the extant Númenórean colonies into the Kingdom of Arnor and Gondor. By the middle part of the Third Age, almost all of Middle-earth was part of this benevolent empire—and although Arnor was eventually destroyed thanks to the machinations of the Witch-King, a diminished but potent Gondor survived and led the resistance coalition against Sauron for another 15 centuries—until its glory was restored and it was reunited with its northern cousin, Arnor, under Aragorn’s reign.
3) Many types of political systems existed in Middle-earth—not just monarchies.
Over seven millennia of recorded history, Tolkien’s world was home not just to monarchy, feudal (Gondor and Arnor, Rohan) and otherwise (Erebor, Moria), but also despotism (Númenor under Ar-Pharazôn), aristocracy (Rivendell), oligarchy (Blue Mountain Dwarves), benevolent dictatorship (Lórien), representative democracy (the Shire’s mayoralty), nomadic chiefdoms (Easterlings), and theocracy (Sauron’s Mordor and the First Age dark empire of his mentor, the satanic Morgoth). In general, the good polities (men of Gondor and Rohan, Elves, Dwarves and Hobbits) possessed not just functionality and rule of law but at least some measure of accountability to subjects, whereas the bad ones were usually simply utilitarian, whether narrowly and basely (Orcs) or somewhat less so (Easterlings, Haradrim).