Personally I think its ok for instances to delete older posts to save space provided that there are means to archive threads that users find valuable. For fediverse to thrive it should be as easy as possible for people to setup and manage instances without having to think about the storage space too much.
Archival of historical content is something that I feel should be handled separately.
Yeah, I’ve never even heard of anyone using imessages here in Finland. It’s mostly Discord or Whatsapp like you said and then maybe Teams for work and Signal with those few more tech savy friends.