Lemmy and other associated link aggregation and forum platforms in the fediverse have the potential to grow sustainably post initial spike by supporting free speech, decentralization, volunteer moderation, federation, community control, open-source development, and alternative algorithms, among other strategies.

1. Free Speech and Content Diversity: Often, users flock to decentralized platforms due to unwelcome changes in content policies on conventional platforms. Thus, Lemmy and its peers should continue to promote the principle of free speech while balancing the need for a healthy, respectful community environment.

2. Decentralization: Decentralization is the cornerstone of the fediverse, allowing each server to function independently, providing a safeguard against platform-wide shutdowns or changes that might displease users.

3. Solid Moderation Policies: The growth and sustainability can be supported by establishing firm moderation policies and a defense against malicious activity. Volunteer moderators can play a crucial role in keeping day-to-day discourse civil and productive.

4. Federation: Embracing the fediverse's interconnected system is critical. Federation allows independent servers to communicate with each other, exponentially expanding users' access to content and communities.

5. Community Ownership and Control: An open-source approach where users contribute to the development and evolution of the platform keeps the platform evolving according to its users' needs and trends.

6. Algorithm Transparency: Unlike major platforms that use secretive algorithms, these platforms should lean towards more transparent and user-responsive algorithms that promote diverse content rather than a concentration on popular content.

7. User Experience: Investing in user-friendly interfaces and attractive designs will enhance user experience and improve adoption, leading to sustainable growth.

8. Community Collaboration: Build channels that enable user involvement, such as voting on platform changes, contributing to development, handling moderation issues, and more, which fosters feelings of ownership and promotes long-term user retention.

9. Promote Local Communities: Encourage local instances based on local communities, specific topics, or languages. These communities are the backbone of any social network and play a significant role in user retention.

10. Partnerships: Build relationships and collaborate with other fediverse platforms to foster interoperability and seamless user experience, making the decentralized platform more appealing to new users.

Overall, the key to sustainable growth post initial user influx is a nourishing, democratic, transparent, and evolving platform ecosystem that is responsive to user needs, while being attractive and easy to use for newcomers.