Firms are intensifying their involvement with open source software (OSS), going beyond contributing to individual projects and releasing their own core technologies as OSS. These technologies, from web frameworks to programming languages, are the foundations of large and growing ecosystems. Yet we know little about how these anchor sponsors shape the behavior of OSS contributors. We examine Mozilla Corporation's role as incubator and anchor sponsor in the Rust programming language ecosystem, leveraging data on nearly 30,000 developers and 40,000 OSS projects from 2015 to 2022. When Mozilla abruptly exited Rust in August 2020, event-study models estimate a negative impact on ecosystem activity: a 9\% immediate drop in weekly commits and a 0.6 percentage point decline in trend. We observe an asymmetry in the shock's effects: former Mozilla developers and close collaborators continued contributing relatively quickly, whereas more distant developers showed reduced or ceased activity even six months later. An agent-based model of an OSS ecosystem with an anchor sponsor replicates these patterns. We also find a marked slowdown in new developers and projects entering Rust post-shock. Our results suggest that Mozilla served as a critical signal of Rust's quality and stability. Once withdrawn, newcomers and less-embedded developers were the most discouraged, raising concerns about long-term ecosystem sustainability.