Russian President Vladimir Putin can afford to cut off natural-gas exports to Europe thanks to ample revenues from other commodities, but such a move would come with longer-term risks for Russia’s sanctions-stricken economy and its prolific energy industry. Earlier this week, Moscow resumed gas supplies via the Nord Stream pipeline after annual maintenance ended, easing worries in Europe that Mr. Putin would further crimp exports at a time when the continent is filling up its storage ahead of winter. The restarted pipeline, however, is pumping at just 40% capacity, after a separate, still-unresolved technical issue that Moscow blames on Western...