Historically a state monopoly on the use of force was very rare - in Europe you have the Roman Empire, then it's not seen again until the rise of the modern nation-state from the 16th century on. The feudal system for instance certainly does not depend upon a monopoly on the use of force, quite the reverse. Sometimes D&D mixes up modern and feudal elements, eg Gygaxian D&D seemed fond of mass armies of weak mooks. IRL mass armies were a feature of the modern state, they make no sense in most versions of D&D, where small forces of levelled characters are far more powerful. OTOH small forces of well equipped & levelled Fighters backed by Clerics & Wizards (depending on class frequency) makes good sense, at least in non-3e editions. A baron's entourage should essentially resemble a PC party; a king's entourage should resemble a high level PC party.
Adventurer parties can be readily incorporated into a feudal model; they may be 'questing knights' who swear allegiance and receive magical items, wealth etc in return for pledged service, or they may be independent mercenaries who also receive magic & wealth, but on a contract basis. In either case they are simply one power source among many in the feudal structure, not much different from another baron, a bishopric, a town, a guild etc. They don't need to be disarmed or destroyed if they are willing to work within the system.
A 15th century knight in plate armour vs a single unarmoured man with a melee weapon or bow (anything except a firearm) is pretty much just as safe as a D&D mid-level adventurer vs a 0-level schlub. In D&D, rulership will either be by small groups of tough characters, much like real world feudal system, or if mid-level power is rare but there are numbers of high-level supermen you might see a "Way of the Exploding Fist" structure where rulership is by lone super-powered heroes who can take on armies. A powerful leader may bring together numbers of these supermen for particular endeavours, which is rather what The Iliad looks like...
What if a dragon or other monster too powerful for the local ruler appears? It is a problem if you are thinking in terms of modern nation states who claim legitimacy via monopoly on the use of force, but not otherwise IMO. In a feudal system, if the dragon is active then the system will attempt to incorporate the dragon as another power node - send it tribute, grant it a title, etc. A hostile dragon will be destroyed if possible, if not then it will be ignored as far as possible, if neither is possible then other power nodes will ally against it, and offer rewards for its destruction - just as in a typical D&D adventure. The adventurer party who kills the dragon and gets the reward then becomes a recognised power node and part of the system. What if the dragon kills the local ruler? If it takes over it becomes part of the system. If it just kills & destroys without taking over, and cannot be stopped, then it's no different from a plague, earthquake or other natural disaster.
One reason I really like the Wilderlands of High Fantasy setting is that it has a power-node setup which perfectly accommodates powerful PC groups, monsters etc - it looks exactly like what you would expect when there are super-powered entities and no one can claim a force monopoly much further than their own reach. Conversely settings with modern-looking states plus super-powerful individuals break easily; 3e Greyhawk is a good example, as is any setting based on the world-building advice in the 3e DMG combined with characters created using the 3e PHB... Pathfinder/Golarion also has this problem, only slightly alleviated by a different level distribution than in 3e - more mid-level NPCs, very few level 15+ NPCs - and IME requires a lot of suspension of disbelief, where Wilderlands needs none.
D&D was originally written with the assumption the PCs likely would take over. The power curve reflects this. Wilderlands is a good setting written with this assumption. Published settings and campaigns that are not written to accommodate this (Golarion, and, weirdly, Greyhawk!) definitely have problems. Most Golarion-set Adventure Paths would better fit a shallow power curve system like Savage Worlds, not D&D/Pathfinder. Forgotten Realms is mostly closer to Wilderlands than to Golarion, but if a campaign like Princes of the Apocalypse is written with 3e/Paizo-style assumptions of PC passivity then it is going to be a problem. My suggestion is that as PCs rise in power, be sure to have them offerered positions within the existing power structure.
Think about your game's in-world society not as a Hobbesian state monopoly on violence, a pyramid structure with the Sovereign on top, but rather as a network of power nodes, a linked system, with many of those nodes potential independent sources of violence. If you do this then you can understand the place of PC groups much easier - and that place may well be on top of the system! But if the ruled don't accept your rule, you're not a ruler, you're just a monster. And even powerful monsters tend to get ganked eventually.
Uther: "To kill and be king? Is that all?"
Merlin: "Perhaps, not even that..."