Continued contributions of core developers in open source software (OSS) projects are key for sustaining and maintaining successful OSS projects. A major risk to the sustainability of OSS projects is developer turnover. Prior studies have explored developer turnover at the level of individual projects. A shortcoming of such studies is that they ignore the impact of developer turnover on downstream projects. Yet, an awareness of the turnover of core developers offers useful insights to the rest of an open source ecosystem. This study performs a large-scale empirical analysis of code developer turnover in the Rust package ecosystem. We find that the turnover of core developers is quite common in the whole Rust ecosystem with 36,991 packages. This is particularly worrying as a vast majority of Rust packages only have a single core developer. We found that core developer turnover can significantly decrease the quality and efficiency of software development and maintenance, even leading to deprecation. This is a major source of concern for those Rust packages that are widely used. We surveyed developers’ perspectives on the turnover of core developers in upstream packages. We found that developers widely agreed that core developer turnover can affect project stability and sustainability. They also emphasized the importance of transparency and timely notifications regarding the health status of upstream dependencies. This study provides unique insights to help communities focus on building reliable software dependency networks.