The last 911 powered this way vanished after the 1998 model year, while the air-cooled Beetle soldiered on until 2003. Between them are, arguably, the most iconic car and the most popular car every made.
The video above compares air-cooled engines to water-cooled. While the latter has more weight, potential leaks and complicated parts, it has one key offset to all of that: The ability to meet tightening emissions standards while delivering levels of power that would leave an air-cooled engine seized on the side of the road.
Because their cooling systems are generally always "on," even when stone cold, air-cooled engines tend to run cold longer at startup. That spews more emissions from partially burned fuel, which is a great way to get on the wrong side of the US EPA in the 21st century.
At the same time, cars have been getting bigger, heavier, faster and more fuel efficient, an almost impossible combination of demands that are only met by engines designed to run hotter. Water cooling brings a remarkable set of components to an engine that prevent it from melting down under such demands; air cooling just has fewer levers to pull to manage heat.
That said, air-cooled 911 engines were actually fluid-cooled in one key way: Their oil was always routed to a cooler to strip off some heat, a process made more effective by their dry sump designs that move oil under pressure. But the real pressure was the approach of engine design modernity.