Throwing away your community’s history doesn’t really make sense unless unless whatever people said last year doesn’t matter at all. Slack deletes stuff that’s some months old by default, I think.
How big is your database? Is your cost issue about Disk space? There are cheaper ways to get more of it (e.g., pushing images to S3). If the database is very big, then indeed, you can need more RAM to support it.
Are you having performance issues? Is the forum slow?