I can't envision a federated service dying

The only way Lemmy could "die" would be if it is surpassed by a similar project (like Sublinks, Piefed, Mbin). If that project is also part of the fediverse, that wouldn't necessarily be a bad thing.

Edit: I suppose another way Lemmy could die would be if one of the larger instances somehow makes a closed-source fork of the software and improves it to the point where the majority of users flock to it. My hope is that Lemmy is already decentralized enough that this couldn't happen, and that Lemmy users would be savvy enough to oppose this if it did.