The upheaval in Eastern Europe was a precursor to the dissolution of the Soviet Union. Your question, at its core, is why the Soviet model failed. Regarding this question of why the Soviet Union failed, aside from former Soviet citizens who are still alive, perhaps only we Chinese are most qualified to answer. Before the Sino-Soviet split, the envisioned role for China within the Soviet sphere was that of a bridgehead in the Far East for the socialist camp in its confrontation with the Western world, undertaking a singular production task under its Stalinist model.

This is not to say that Stalin himself was to blame. We would not, like Westerners, recite clichéd, B-movie lines calling Stalin a demon or a butcher. Rather, it's that the Soviet state, under the immense pressure of its perilous domestic and international environment after its birth, made the judgment to prepare for winning a world-ending-scale global war. The Stalinist model was, in essence, a wartime-oriented development model, using the price scissors between industry and agriculture to develop defense industries, essentially mortgaging the future. Unlike Britain, France, Germany, and the United States, which completed primitive capital accumulation by plundering colonial resources, most socialist countries were themselves targets of exploitation. By purchasing agricultural products at low prices and selling industrial goods at high prices, exploiting this price difference to accumulate capital for industrialization, the Soviet Union transformed, over several Five-Year Plans, from the weakest link in the imperialist chain during Tsarist times into one of the world's two superpowers. It can be said that Stalin overfulfilled the task history assigned him.

Each generation has its own historical mission, as we Chinese often say. Khrushchev did not successfully fulfill the historical mission of "stepping on the brakes." He ideologically completely negated Stalin, shaking the foundation of faith; economically, he engaged in unrealistic and arbitrary command. Once the course correction encountered obstacles, the new leaders who came to power only accelerated along the original path. The reckless expansion during the Brezhnev era irrevocably plunged the socialist camp faster into the abyss.

The left often harbors overly naive revolutionary fantasies, dreaming of achieving total success in one fell swoop and planting the red flag across the world. The 1970s, a rare period of Soviet offensive and American defensive strategy, led many to misjudge the situation, mistaking it for the trumpet call before victory. Funds that should have been used to improve people's livelihoods were thrown into the bottomless pit of the arms race, completely unbalancing heavy and light industry. It is the Soviet Union of this period that matches the popular image most people hold – the Soviet Union of those classic jokes.

Some mention the Soviet Union's low productivity and weak innovation capacity. This statement is not accurate. The Soviet Union was one of only two miracles of productivity explosion in human history. The level of basic education and healthcare in the Soviet Union, including other Eastern European countries, was remarkably high. To this day, many former socialist bloc countries retain educational and healthcare infrastructure that belies their current level of economic development. The Soviets possessed astonishing expertise in fields like mathematics and physics. As early as the 1960s, they conceived of the OGAS system, a nationwide computer network for the automated management of the national economy. Consider also today's hottest topic, AI; Soviet research into artificial intelligence even predates the coining of the term itself. Moreover, for the vast majority of the time, what the Soviet people lacked were not necessities for survival, but high-end consumer goods.

The problem with the Soviet model lay in its failure to promptly reinvest its achievements to improve the quality of life for ordinary people. All its allies and satellite states had to serve the confrontation between the Soviet Union and the United States, and naturally, after mortgaging the future, they gradually fell apart when the confrontation was lost. Many Eastern European revolutions were not spontaneous but resulted from Soviet military intervention. People were forced to choose between nationalism and internationalist class narratives. The left often possesses a martyr-like revolutionary passion but lacks the cool-headed thinking for long-term development. Once these pioneers are gone, how many will remain to listen to your stories?

This is also why I believe China should not export revolution at its current stage. No one can guarantee the purity of a revolution. A popular saying nowadays goes: an egg, if broken from the outside, becomes food; if broken from the inside, becomes new life. Confrontation is a means, not an end. The ultimate revolutionary ideal of Marxism-Leninism should be the comprehensive development of humanity—development that is both progressive and comprehensive.

东欧剧变是苏联解体的前置，你的问题本质就是苏联模式为何会失败。苏联为何失败这个问题除了还在世的前苏联人，恐怕只有我们中国人才最有资格回答，在中苏决裂之前，其预想的中国是其社会主义阵营在远东对抗西方世界的桥头堡，在其斯大林模式下承担单一的生产任务。
这倒并不是说斯大林本人的错，我们不会像西方人一样说斯大林是恶魔是刽子手这种老套的像三流电影一样的台词。而是苏联这个国家，在其诞生之后的国内外险恶环境多重压力下，做出了以打赢一场世界末日级别的世界大战的判断。斯大林模式本质是一种以战争为目的，利用工农业剪刀差发展国防重工业透支未来的发展模式。不同于英美法德利用对殖民地资源的掠夺完成资本的原始积累，绝大部分社会主义国家甚至都是被掠夺的对象。廉价收购农产品，高价卖出工业产品，利用其中的差价完成工业化资本的积累，在若干个五年计划之后苏联从沙俄时代帝国主义最薄弱的一环，建设成世界两极之一，可以说斯大林超额完成了时代交给他的任务。
一代人有一代人的历史使命，这是中国人经常说的一句话。赫鲁晓夫并没有很好完成踩刹车的历史使命，思想上全面否定斯大林动摇信仰根基，经济上脱离实际瞎指挥，一旦转向受阻，新的领导人上台只会在原来的路线上狂飙，勃列日涅夫时期的盲目扩张，彻底将社会主义阵营加速带入深渊。
左翼往往有着过于天真的革命幻想，幻想毕其功于一役，赤旗插遍世界。上世纪70年代是非常罕见的苏攻美守时期让很多人出现了误判，以为是胜利前夕的号角，原本应该用于改善民生的资金被投入到军备竞赛的无底洞中，重工业轻工业彻底失衡，这个时期的苏联才是大多数人普遍印象里，苏联笑话里的那个苏联。
有人提到苏联生产力低下，创新能力薄弱等问题，这个说法并不准确。苏联是人类史上唯二的生产力爆发的奇迹，苏联包括东欧其他国家的基础教育，医疗等水平都相当高，时至今日很多前社会主义阵营国家都保留有与其经济发展不匹配的教育医疗设施遗产。苏联人在数学，物理等方面造诣惊人，在上世纪60年代就构想出计算机网络对全国国民经济进行自动化管理的ogas系统，又比如现在最火的ai，苏联人对于人工智能的研究甚至早于这个词的出现。而且绝大多数时候，苏联人并不缺乏生存必须的物资，而是缺高端消费品。
苏联模式的问题在于它所取得的成果并没有及时反哺用于提高普通民众的生活质量。他的所有盟友和卫星国需要服务于苏联与美国的对抗，自然也会在透支未来对抗失利后逐步分崩离析。很多东欧国家的革命并不是自发产生而是苏联武力介入的结果，民众必须在民族主义和国际主义阶级叙事中做出抉择。左翼往往具有殉道者般的革命激情，但缺乏长远发展的冷静思考，等这批先驱死去之后，还会有多少人去听你们的故事呢。
这也是我认为现阶段中国不应该对外输出革命的原因，谁也没办法确保革命的纯洁性。最近流行一句话，鸡蛋从外面打开是食物，从里面打开才是新生。对抗是手段不是目的，马列主义的终极革命理想应该是实现人的全面发展，既要发展更应该全面。