NieR: Automata surpasses 6 million units shipped, NieR Replicant ver.1.22474487139... surpasses 1 million
NieR: Automata reached 5.5 million units just a few months ago, so it seems like broad interest in the series has allowed the game to continue to sell well more than 4 years since its release.
As a point of comparison, NieR: Automata reached the 1 million mark in April 2017, noting the title had a staggered release across platforms around the world.
Square Enix has recently promised an upcoming patch to the Steam version of Automata, hopefully looking to fix some of the long-lasting performance bugs that have plagued the release since launch. We're still waiting to see how that might turn out in the future.