I can only give a perspective from the era of my study but I'm not aware monarchical dynasties collapsing into civil war is unusual in other cultures.

The Mandate of Heaven was important but I don't think in the way you seem to think. Every dynasty sought to make clear it had the mandate, the perception that a dynasty had lost it was damaging, it was a major part of legitimization, prophecy and politics (particularly when things were not united), the three kingdoms saw new situations so understanding of the mandate had to be evolved to justify the changes.

The debate about who has the mandate between Han and Jin (Wei and the Cao dynasty or the Shu-Han of Yi) shaped the way history was written. The morality part of the mandate of heaven shaped people's beliefs and the way history is portrayed as the writers warned whichever Emperor they were writing for by using fallen dynasties as a lesson (don't be too interested in ladies, don't listen to eunuchs, support my faction, show restraint, that sort of thing) or Emperors used history writing that following the example of the Sima rise would not go well.

I can't, in my area of understanding, think of any Empire (Han, Wei, Wu, Shu and Jin) were the cause of their fall was the mandate of heaven. In a dynastic rule, rulers can be killed and their dynasties fall or are overthrown. This might be normal but how does that work with the idea of the Son of Heaven, the representative of the Heavens who is needed to keep the cosmic balance, the divine sovereign (who you may or may entirely listen to)? The mandate of heaven helps explain that change, how thinking justified "out with the old ruler, in with the new" and changing support.

Population shifts and the large size of China (though not the size we know now) did have impacts on various things. The south's growth of Han-Chinese population during the Han and flights there during the civil war allowed the Sun family to found a sustainable empire which lengthened the three kingdoms civil war and provided routes for other dynasties. Wei's failure to unite the land because of it then raised doubts about its full legitimacy and led to questions about their moral character. The loss of Han-Chinese population on the very northern borders became a problem for the latter Han in its fights to maintain control against tribes like the Qiang while the vacuum left would become a problem for the Jin dynasty as the foreign powers would take advantage of Jin's internal problems.

There were regional identities and rivalries but, at least during the Han and after, they recognized themselves as one nation. There were recognized frontier regions where their natural wealth, distance from the central plains and natural barriers could be used to establish a regime like Sichuan or, from Sun family onwards, Wu. A bigger problem however tended to be not too large or too populous but that regimes struggled to get full control of their populace and their resources. That the wealth and manpower were mostly in the hands of the power families with central authority and support whittled away bit by bit. If you don't have the resources to deal with the problems and having to persuade or cajole the powerful families for help, it does take its toll.

The bad rulers are the sort of narrative that was part of the mandate of heaven and the moral lessons for future Emperor's. It is Emperors Huan and Ling with their love of the eunuchs leading to the Han decline, it is the last Emperor tropes that can be seen in likes of the last Wu Emperor Sun Hao. Things tended to be rather more complicated than that, that one or two bad rulers was not usually why things went wrong.

Let us say, for the sake of argument, you as a minister felt the current and adult Son of Heaven was sending the dynasty down a bad path... you might find it difficult to change him. First, you need to gather sufficient support and if any guess wrong who might back you, then your act of treason would be exposed and you would likely be dead. Not everyone may agree that the Emperor is bad (then or now), others self-interest will be aligned with the Emperor, others may not be a fan of the Emperor but see the idea as wrong morally and/or dangerous for stability.

You gather enough support at court, what guarantees an Emperor will go quietly? Some did when their plots failed or when they knew the dynasty was doomed. Others gathered support and launched counter-coups which, for the Han, resulted in the deaths of the controllers and in the case of Wei Emperor Cao Mao it failed and though he died in the streets, it was hugely embarrassing for the controlling Sima family to the point they sought to suppress the regicide in the texts.

The Emperor is going to quietly then what about the generals? Will they accept this or will they act on behalf of the Emperor to restore him? Which relative are you going to choose and can you be sure the candidate would agree to the job? Can you guarantee the candidate you have picked will get it or might you be outplayed in the court? Do other relatives, as in the Jin dynasty, have their own armies and positions where they may fight to be Emperor instead of your chosen candidate? How sure that your chosen candidate isn't going to be a disaster?

You might be well-intentioned but how do you stop future controllers of an Emperor using precedent to replace the Emperors for less benign reasons? Will people accept you were doing it for the dynasty or will you be seen as a dangerous controller abusing power? How much damage to the legitimacy and authority have the Dynasty have you done?

That isn't to say changing Emperor can't happen. The Former Han actions of Huo Guang and his granddaughter Dowager to depose Liu He as the successor of the late Emperor Zhao provided a precedent for deposing misbehaving/"I would kindly like my power please" Emperor's before they came to full power. It didn't always end well, Dong Zhuo deposing Emperor Bian helped start the three kingdoms civil war and a lot of the problems I listed were shown in the 3kingdome era with attempts to change rulers, but it could be done. Though it tended not to signal a dynasty in rude health, instead the dynasty being either in desperate straits or under the thumb of someone rather more powerful.

Each dynasty will have had its reasons for falling apart, new threats its predecessors didn't face, attempts to fix problems of the past creating entirely new problems they didn't foresee, reasons why central authority fell away and power-balances grew out of control. The reasons the Han declined then collapsed differed from those kingdoms of the civil war that fought unsuccessfully to replace the Han (and their reasons for their failure and end differing from each other) or the fall of Jin that united the land then collapsed so soon afterwards.

I can go into the collapses of the Latter Han, Wei, Wu and Shu in more detail if it helps or answer any questions

Edit: Forgot sources

Empress and Consorts by Robert Cutter and William Cromwell

SGZ by Chen Shou, annotated by Pei Songzhi, translated by Yang Zhengyuan including Annals of Wei

The Three Kingdoms and Western Jin A History of China in the Third Century A.D and Fire Over Luoyang by Rafe De Crespigny

What I meant by the title was basically 'Why Chinese dynasties always break up after a few hundred years at most?'