We don’t know how the fediverse would play out in the long term but I expect a few things:

• Lemmy devs will implement locking old posts in the future
• instance owners will decide on their post locking policies
• Many instances will not survive for as long as Reddit survived till today
• Third party archivers (perhaps archive.org among others) will need to step in and archive posts. Ofc archived posts is not interactible.