Bitcoin hasn’t found its killer app, but that might not matter. In early September, one Bitcoin was worth almost $5,000. Then the Chinese government cracked down on cryptocurrency investments, and Bitcoin’s value plunged 40 percent in a matter of days, reaching a low below $3,000. But Bitcoin bounced back. By early November, one Bitcoin was worth almost $8,000. Then last week, a controversial effort to expand the Bitcoin network’s capacity failed.