Foreign Affairs

Their being cut off from the world banking and trade system allows them to, at least internally, rejigger as needed as the normal economic guardrails do not exist for them. As long as they can feed themselves and keep the lights on, however limited the food choices and despite some brownouts, Putin can likely remain in power. Also assumes the vodka keeps flowing but if you have grain for bread the distilleries can go full tilt. North Korea has been in this or an even worse economic pit for decades and yet somehow survives to be a thorn in Asia's side and so we have Putin's Russia on the European side of things. Iran is similar too.