NOMA has been recognized as a highly promising FRA technology to satisfy the requirements of the fifth generation era on high spectral efficiency and massive connectivity. Since the EE has become a growing concern in FRA from both the industrial and societal perspectives, this article discusses the sustainability issues of NOMA. We first thoroughly examine the theoretical power regions of NOMA to show the minimum transmission power with fixed data rate requirement, demonstrating the EE performance advantage of NOMA over orthogonal multiple access. Then we explore the role of energy-aware resource allocation and grant-free transmission in further enhancing the EE performance of NOMA. Based on this exploration, a hybrid NOMA strategy that reaps the joint benefits of resource allocation and grant-free transmission is investigated to simultaneously accomplish high throughput, large connectivity, and low energy cost. Finally, we identify some important and interesting future directions for NOMA designers to follow in the next decade.