Those regimes were equally untenable. The Nazis only lasted 10 years. The Khmer Rouge held power for just 5. Maoism and Stalinism died with their namesakes. By comparison, Imperial Russia lasted centuries before the peasants got tired of their shit.
Nazis: If they had better economics they'd have done just fine. Their greatest Achilles heel (in a victory/negotiated peace scenario) was their conquest economy, everything else could be maintained just fine. Sad as that is to say. I suppose that does make them untenable though. So that's one.
Khmer Rouge: I don't know too much about the Khmer Rouge, but I will say as extension of the above, that outside intervention was necessary to end it. It might have exhausted itself (that's an insane amount of social disruption) but whether that exhaustion comes before or after the extermination is "complete" would be my question.
Maoism: Only died because of internal machinations within the Chinese Communist Party. Maoism of a sort was definitely tenable, it wouldn't have been pretty, but so long as they stayed away from Cultural Revolution era policies, they'd be fine.
Stalinism: Likewise, only died because of internal politics in the governing party. Internal politics that, I should add, were not one sided. Khrushchev wasn't preordained to become Premier anymore than Deng Xiaoping was, probably less so. A few personnel changes in the party can have Stalinists remain in power indefinitely. The death of Stalinism was one of those happy instances where relative decency wins out.
The last two are definitely not untenable systems, they were still vulnerable though.
The first two I'm more open to agreeing with you on, but I don't know enough to guess how the Nazis would hold up in peace time. Same and to even lesser extent, with the Khmer Rouge.