Historically, Communism does not have a good track record. Usually, the consolidation of so much power has led to corrupt leadership. That was the case with the Russian Revolution and their experiment with Communism as well as the Animal Revolution and their experiment with Animalism. The initial philosophy of Communism is that people would control the means of production (rather than a few reaping the surplus labor of the masses). The idea of Communism is, in theory, not inherently bad. The idea is that profits will be distributed equally. However, when put into practice in the book and historically, the severe limitations on the function of a free market negatively affect the economy and the consolidation of power to a centralized state seems to always result in a deterioration of citizens lives rather than an improvement.
Even Snowball, the virtuous rebel trying to carry out Old Major's dream, agrees that the pigs should control the milk and apples. And when Snowball, the only strong voice of reason, is gone, Napoleon consolidates power more and more. Then, little by little, he abandons the spirit and the economy of Animalism, retaining more power and eventually trading with the other farmers with whom the Animals were sworn enemies.
It does seem that Orwell is making the argument that Communism is not sustainable. (One current example is China. Although they have established themselves as a superpower, there are still problems because of the constraints of their communist system.) In the novel, in Chapter 7, Squealer marks a significant moment when they turn their backs on the Rebellion. Squealer communicates that they've achieved the ideal society when in fact, that society has already been deteriorating, far beyond the original spirit of the Rebellion.
"It's no longer needed, comrade," said Squealer stiffly. "Beasts of England was the song of the Rebellion. But the Rebellion is now completed. The execution of the traitors this afternoon was the final act.
That being said, the novel is about socialism in addition to communism. Orwell initially was interested in socialist ideas but became disillusioned when he saw them put into practice; particularly with Russia as it applies toAnimal Farm. However, socialist programs within democratic governments have been successful. Things like social security, police forces, firemen, and Medicare are all social institutions run by democratic governments. So, although socialist governments have also had bad track records, some social programs (within democracies and republics) have been very useful.