Rules TBD.
Stalin was initially receptive to Lenin’s New Economic Policy. I’m not familiar with the whys & whos of its retirement.
I always assumed the serial crises and upheaval of WW2 and the Cold War put experimental policies like that on the back burner and they just never got picked back up.
That's been my understanding as well, the NEP was abandoned because there was a realization that the USSR would not be able to withstand a war with Germany without rapid mass industrialization.
And the state of emergency was very hard to lift after given that the US got all the Nazis to work on sabotage against the Eastern bloc, not to mention had developed a nuclear weapon and refused to rule out a first strike against the USSR.
Exactly, USSR was under siege from western capitalist powers throughout its whole existence.
The reality is that Deng Xiaoping was a classical Marxist so he wanted China to follow the development path of classical Marxism (grasping the large, letting go of the small) and not the revision of Marxism by Stalin (nationalizing everything), because Marxian theory is about formulating a scientific theory of socioeconomic development, so if they want to develop as rapidly as possible they needed to adhere more closely to Marxian economics.