No economic system is sustainable or unsustainable. They just don't pertain to that. Concerns of sustainability requires ethics or judicial system. If anything, socialism is less sustainable than capitalism because it eliminates distributive injustices that cause a reduction of productivity. For example, capitalists would hoard land to seek ransoms in exchange of its access. This causes land unaffordability and under usage. If socialism eliminates that, more land might be used and there might be more environmental destruction as a result. So you need a system that protects land from environmental degradation. Socialism doesn't do that by itself. You need specific laws to protect the environment.

We want socialism because it's more fair and isn't literal extortion like capitalism is, not because it's more sustainable. We also want sustainability, so we want a strong and fair judicial system to go with our social economic system.