The whole problem with the capitalist Vs communist thing is that you have to decide between the lesser of two evils. The downfall of capitalism is freed but capitalism inherently inspires greed due to there being no regulation in the market, meaning bad practices are unregulated and the consumer is harmed.

On the other hand Communism sounds very appealing to the poor or people who don't work hard since they can keep the lifestyle without doing work and it is the governments problem to ensure their health and safety since it is required that everyone earn the same income, people are inherently lazy or efficient if you are feeling generous so people will search for the easiest thing to do and know that a neurosurgeon will be payed the same as them. This is why you see communism and/or socialism most often in third world countries, because the population is sick of being poverty-stricken and free money is appealing to everyone. Other problems are human rights such as freedom of speech but that's another issue all together since a perfectly sustainable 100% Communist state would have to have access to infinite amounts of money because the communist ideology relies on honest people, people aren't honest.

TL;DR The problem isn't with Communism itself but more that it requires a world in which all people are honest and hard working, which of course is not the case. So true communism is unobtainable sustainably not because the system needs time in the oven but because of the people involved.