Instances can grow fast, and that can be a problem if you have limited disk space. If you don't mind losing history that your users have not interacted with, LPP can help. It will purge posts along with their comments and media before a certain time period if no comments, likes, or saves exist for users on your instance.
I feel like this ruins the aspect of an archive of information where users can go back through and find useful info similar to SO in a way. Maybe there will be a meta search engine for looking through all of the popular instances?
We need a sort of Pushshift for Lemmy. That way Lemmy instances can purge posts for their own survival, while a full archive exists and can be accessed if needed. Best of both worlds. Perhaps the Lemmy project should be working with the Pushshift people to integrate that functionality.
It's a balance between losing information due to purging old stuff, and losing information due to an instance shutting down entirely from costs being too high.