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As opposed to being conquered, collapse is typically caused by economic collapse; people stop obeying the law in order to get food, shelter, and what they need to survive.
The economic collapse can have multiple single causes, or a combination of them. Drought and subsequent crop failure (or unstoppable crop diseases like the Irish potato blight) can cause it; a drought of a few centuries long apparently had a lot to do with the collapse of the Mayan empire.
Rampant corruption and greed from the people in control can cause an economic collapse or societal collapse: In the latter, nobody trusts the law and takes the law into their own hands; or nobody trusts the money any more and refusal to accept it leads to a barter society.
Leaders of empires and their generals are typically less than 1% of their adult population: See here for modern America: How Many Elected Officials In the USA?
The answer: About 520,000, going all the way down to the local school district people and small town mayors and city councils. But there are about 245 million adults in the USA (with 74 million under 18), so 520K is less than 1/5th of 1%.
Now that doesn't include military or police, but the point is simple: All the government, military and police are overwhelmed about 100 to 1 by citizens, and this is true in an empire, too. If the leaders of the empire cannot keep their citizens fed, sheltered and out of desperation, the people will not necessarily revolt and establish a new government, they may just walk away and stop listening, and when needed respond with violence to coercion trying to take taxes or services.
That is what collapse without revolt looks like, people walking away, forming their own groups and breaking away, and in large extent the military and police just joining them and defending them. Because when it comes down to it (and I have been in the military) soldiers and police have to eat and feed their families too, and in an economic collapse they may join some co-op and contribute their efforts to that instead of the failed government.
I have read about collapse before, and cannot remember the details, but the solution was not something I thought of: people rallied around large farm and ranch owners for a fair share of the production and trade; agreeing to labor on the farm and protect the farm from predatory humans that would try to steal the food and/or kill the workers. The perfect job for cops and soldiers that don't want to join in with predatory and criminal gangs. Farm owners became basically mayors that dealt with other farms in trade for their products; potatoes for chickens or beef. People had to work hard on the farm because the collapse meant there was no power for the big equipment, they went back to farming, weeding, irrigating and harvesting by hand; not to mention squads that went hunting for the group to bring home wild food.
Added: I suppose the same idea, for a coastal area, might work for smaller fishing boats that could be operated manually. People need protection and have to work, the fisherman with help can supply a ready food supply but needs protection and muscle to get it all done.
The idea is that, in an economic collapse, most people revert to a "will-work-for-food" mindset. Of course some jump straight to a "will-kill-for-food" mindset, but the majority of us aren't capable and don't give ourselves much of a chance of prevailing with the latter mindset; so those willing to work will try to find a way to be useful to somebody that is already a food producer with the knowledge and experience (and perhaps some manually powered equipment) to do that successfully.
I might suggest reading up on the history of the Roman Empire. A good example of an historical empire falling from within. If you're in the US, you can also have been watching the nightly snooze for last 30 to 50 years to see it happening in real time, and some of the late efforts to prevent that from happening and perhaps stem that tide.
You are right that small "heroic" groups don't generally get the job done. What brings an empire down is almost always inner heart rot. Cultural decay, lack of civic enthusiasm, inner culture conflict & turmoil, social engineering. The majority may become tyrannized by a radical minority and eventually they will be unable to say anything against the radicals for fear of being labelled "racist" or "bigot" or "homophobe" or whatever other nonsense charges the radicals decide to level.
Truth becomes a commodity and those who shout loudest and who wave all the appropriate flags and hoist signs with all the right key words get to make up their own truth and force the lie on everyone else. Open discussion and expression of Truth are quashed. Intelligently applied democracy fails. Authoritarianism rises and pretty soon you're well on your way to failed-statism country.
The only way to ensure that an empire will fully disappear (including culturally) is to destroy its population and knowledge almost completely. Empires tend to re-emerge calling themselves by different names.
In a low tech setting, natural disaster (massive volcanism, crop failures, flooding, etc.), pandemics (with very high mortality, perhaps, up to 90%), and divine intervention are the best options. Pandemics are a bit less effective since they do not tend to destroy cultural heritage such as books, architecture, art, and everyday objects.
In a high tech setting, the disasters can be man-made. It is also possible that an Empire decides to relocate and destroy any evidence of its existence (but technically it will only destroy this Empire in this particular location).
Another option is a conquest by a different culture. This new culture can enslave or assimilate citizens of the empire instead of killing them. At the same time, they need to destroy all cultural artefacts belonging to the empire. This strategy works best with low technological settings, of course.
