So what would the civilized rulers do about cultists? If they are many, then they would wage war. If they are few, then they would hire adventurers. But very frequently the cultists have some doomsday device / plan / agenda to take over the city / country / world / universe. Is this realistic?
Some cultists have impressive resources at their hand e.g. Iuz. Others, not so much. In any case, cultists are a higher threat to whatever society they act upon than e.g. bandits or a thieves guild or bears invading the forest.
So they should provoke stronger reactions, even w/o the adventurers popping in. In the case of Iuz that is not easy, you would need the combined armies of several nations to beat him.
With smaller cults, they should be exterminated quite fast under normal circumstances, just because of their numbers cannot withstand the law for extended times.
Conclusion: The only cults which should exist in-game long term are the bigger ones, the OP is correct, since in official products also smaller cults are often promoted more often then they would naturally occur.
Thats rather the point though - PCs are getting hired to hunt down those minor cults before the grow to be big cults. Thats the premise of the Conan the Barbarian movie and Conan the Destroyer and Indiana Jones and the Temple of Doom and ...
I did a campaign once in which the PCs were all part of the Inquisitors in Mother Church and were charged with recovering Holy Artifacts and hunting down evil cults and heresies
organized crime by its nature is too subtle and stealthy for DnD, its just a whole lot of petty mooks whereas the Big Boss stays hidden and unknown. In contrast baby skilling cultist in league with eldritch horrors are starkly evil and worth killing Even if they are just low level neophyte who havent yet been allowed to join the orgies.
That said I do like killing bandits, especially the despicable kind who rape the feilds and pillage the women.
Serial Killers are another good group to use - Human cannibals lurking in the woods outside of town., although they are often just cultist without the dark powers to motivate them
Organized criminals are nothing more than businessmen with a violent bent. If faced with a tough group of outsiders, they will simply try to buy them off, frame them for something, bribe officials to harass the PCs, or lure them away with false (or real) rumors. They have no reason to stand and fight. They are part of the community, and can simply wait out any storm the PCs can bring, until the PCs leave.
If the PCs (outsiders) start killing locals (criminals) the local authorities are going to take a dim view, especially if they are on the payroll.
Slavers have been used a lot, but they only work if they are in a place where slavery is illegal, and they have a market where it is illegal. If slavery is legal, then the PCs will have a serious problem.
The key is to take a lesson from WH FRPG: cults there are pervasive, and each aspect of Chaos has radically different cultists. Moreover, the cults are part of a larger whole: the cults in society, the bands of beastmen in the forests, the armies of Chaos are all linked.
You're making a great case for the use of organized criminals in games because those all sounds like excellent adventure hooks to me. The cartels in Mexico don't seem to be afraid of a fight when bribes, threats, and other options fail them.
Correct, but any criminal organization not dealing with e.g. assasinations as their prime business case, but rather things like smuggling, trading illegal substances, extortion etc. prefer to avoid direct conflict even if they have the power to do so. Your example is the extreme but a good example of what happens if things get out of control.