An economy cannot work without foreign trade, being blocked from that by a major player essentially does that.
If you look at communist countries that are small enough to be self-sufficient, it tells a different story. Take Cuba, it is blocked from trade with the US and a fair chunk of other countries, however, Cuba is self-sufficient enough that it has not only survived but it has flourished, with a ridiculously high literacy rate, and a universal public health system that puts much of the world to shame.
I have but two observations left to make.
1. If the communist system was inherently a fail-state, then the US would have to do nothing but sit back and wait until it collapses. Instead of that they try relentlessly to sabotage any form of it. They tried, for example, over 600 times to assasinate the leader of Cuba. It's a real shame because it forces the communist parties to be anti-free-press, and anti-free-speech because of the risk of foreign countries abusing that to seed a revolution (Indeed, it was admitted on record that the only reason the CIA hasn't done that in Cuba is because it lacks resources, and Cuba doesn't allow full internet access (as I understand it)).
2. It took over 800 years to establish a stable republic. That doesn't mean that it could not be done, or that the republic was an inherently broken system, but it took a number of 'failures' to make a success, and the right political and societal advancements to be made.