There is a second reason I think socialism is unsustainable. European Socialists are all proponents of political correctness. And PC requires one to put on blinders and ignore the truth.
PC : Islam is the religion of peace.
Truth : nothing could be further from the truth.
Islam is at war with itself and every other religion including atheism. Islam was born in war, rape, looting and human slavery. Those principles remain in place today. Most terrorists today are Islamists with the possible exception of BLM and Antifa which are socialist/communist.
And Socialist Europe through open their borders to Islamists. Angie in Germany thought she was going to get workers to build BMW's. Instead what she got was a swollen welfare roll, rape and crime soared, and Islamists will never assimilate into German society.
Surprise, surprise, surprise.
If the socialist Europeans had paid attention to the truth and not bowed to PC their national debt would be lower, European citizens would be safer, and not so far in debt.