Lower bandwidth for who? When images are cached on other instances, it allows two things:

• Load sharing. The original instance doesn’t have to serve the whole fediverse, but only its own users + 1 request per other lemmy instance.
• Data availability through redundancy. If the original instance goes down, the cached image is still viewable on other instances.

For a more user-friendly (IMO) alternative, altough not open source, there is Resilio Sync. The main difference with Syncthing is that you don’t introduce clients to each other; you just share folders.

When lemmy.world will disappear, that’ll be a lot of communities (and valuable information) that go with it.

However, I have a strange feeling about it not being open source. The recent events with Reddit have only increased this discomfort.

Well, Obsidian is not a live service. If the developers disappear tomorrow or decide to aggressively monetize it, you still have access to the app in its current state, and the whole plugins ecosystem.