Firms are intensifying their involvement in open source software (OSS), moving beyond contributions to individual projects to release their own core technologies as OSS. These technologies, from web frameworks to programming languages, underpin growing ecosystems in which the firms that incubate and support them serve as anchor sponsors. Yet little is known about changes in developer participation and ecosystem renewal following the withdrawal of these sponsors. We examine developer activity and ecosystem renewal following Mozilla Corporation's withdrawal from the Rust ecosystem, using data on nearly 30,000 developers and 40,000 OSS projects from 2015 to 2022. Following the August 2020 withdrawal, event-study estimates indicate a 9% immediate decline in weekly commits and a 0.4 percentage point reduction in the growth trend. We observe an asymmetry in the post-withdrawal changes: activity among former Mozilla developers recovered, whereas other developers remained below projections, with larger shortfalls among those farther from Mozilla in the collaboration network. We also find a marked slowdown in new developers and projects entering Rust post-shock. Our results suggest that Mozilla may have served as an important signal of Rust's quality and stability. Following the withdrawal, newcomer entry slowed and contribution activity among less-embedded developers declined, raising concerns about long-term ecosystem sustainability.