Serverless function-as-a-service (FaaS) platforms offer a radically-new paradigm for cloud software development, yet the hardware infrastructure underlying these platforms is based on a decades-old design pattern. The rise of FaaS presents an opportunity to reimagine cloud infrastructure to be more energy-efficient, cost-effective, reliable, and secure. In this paper, we show how replacing handfuls of x86-based rack servers with hundreds of ARM-based single-board computers could lead to a virtualization-free, energy-proportional cloud that achieves this vision. We call our systematically-designed implementation MicroFaaS, and we conduct a thorough evaluation and cost analysis comparing MicroFaaS to a throughput-matched FaaS platform implemented in the style of conventional virtualization-based cloud systems. Our results show a 5.6x increase in energy efficiency and 34.2% decrease in total cost of ownership compared to our baseline.