The USSR never had a single year of peace, in its entire existence. Following the revolution was a brutal civil war in which 14+ nations (including the US) landed troops to try to stop the republic. Even after the reds won against the whites, they had years of intrigues against them, then rising fascism, which a lot of historians see as a continuous conflict in eastern europe from the years between the twe world wars.

Stalin presciently stated that “we have 10 years to industrialize in the time it took capitalist nations 50+ years, or we’re toast”. Then you have operation barbarossa and the nazi onslaught, with its scorched earth policy and genocidal onslaught of the USSR, the eastern front of ww2 being the bloodiest conflict in history, with the soviets saving the world from fascism.

Then you have the US atom bombing civilians as a warning to the soviets, and 60 years of a cold war arms race, and too many other threats and incursions to count.

The USSR wouldn’t have lasted a single year if they disarmed and followed that advice, and europe would probably be all speaking german now if it weren’t for Stalin.

At what point do you think Marxists believe the DotP is to be ended? Moreover, what do you think a DotP is? People weren’t allowed to dissolve government into small communes because they were invaded by more than 14 Capitalist countries, and in addition the Soviets were Marxists and not Anarchists, they wanted full public ownership and central planning as the goal.

Shouldn’t the dictatorship of the proletariat have been disbanded after the revolution was successful?

This question doesn’t even make sense. The dictatorship of the proletariat doesn’t start until the revolution is successful. When the revolution succeeds it replaces the pre-existing dictatorship of the bourgeoisie with the new dictatorship of the proletariat. These are definitional. The fact that you asked the question means you are missing some critical pieces of information that will make it quite literally impossible for you to analyze anything about history, communist theory, revolutionary politics, and left organizing.

Why were the people not free to self organize into communes of their own design that best reflects their values?

You are describing libertarianism. Under current global conditions, people self-organizing into collectives creates warlords which reproduces feudalism which reproduces capitalism. People self-organizing into communes that best reflect their values is quite literally how we got to where we are today. Prehistoric human communities formed around shared values and splintered along values misalignment. They formed and disbanded and reformed. And eventually the technologies for hoarding became available (generally agriculture) and then conquest became a viable strategy for survival. Those conditions haven’t really changed yet. The point of a socialist transition to communism is actually to collectively organizing human activity to bring about the conditions whereby conquest is no longer a viable strategy for survival. That requires significant reorganization of production and distribution. So far, we’ve seen it takes longer than a century to pull that off.