Would the fall of the Khmer Rouge be inevitable even if Vietnam didn't invade?

Yes. Exactly! In just four years in power (from 1975-1979), the Khmer Rouge murdered 1.5-2 million Cambodians (nearly 25% of Cambodia's population). Among the Khmer Rouge murdered, there were 200,000-300,000 Cambodians of Chinese origin, 90,000-500,000 Champa people (mainly Muslim), 20,000 Cambodians of Vietnamese origin, 20,000 people went to security prison 21 awaiting execution

The above data says many things, the Khmer Rouge killed 10 times more people of Chinese origin than people of Vietnamese origin, but the Chinese communist government still tried its best to support them.

In just 4 years in power, the Khmer Rouge murdered nearly 25% of the Cambodian population. So perhaps within 15-20 years of being in power, the Khmer Rouge will surely kill all Cambodians. At that time, the Khmer Rouge will certainly collapse on its own because there will be no more Cambodians.

Thanks to the Khmer Rouge, Cambodia became an illiterate country and had the highest rate of mental illness in Southeast Asia. They still dream about the self-proclaimed empire that died thousands of years ago instead of staying awake to work and take care of their starving lives.

If you have stupid ideas like Red Khmer Rouge, restore the Khmer Rouge and keep it in Cambodia, never have the illusion of inciting Vietnam and Vietnam will never invade you.