It's hope that eventually different cultures will develop within different communities with similar contents, so if you don't like, say, the culture or mods of one of the news communities, you can just as easily move to another news community with a culture that you felt is better, whereas on reddit, you don't really have that choice.
All of a sudden the admins no longer want to run things and shut it down. All those communities are gone? What happens to my user account?
They are gone, just like normal forums, except for copies stored in instances federated to yours.
Currently, Lemmy, unlike Mastodon, does not have any way to transfer community/accounts to another instance. For Mastodon, most admins made promises to let the users know at least 30 days beforehand if they were ever to shut down their instance, but these should eventually be implemented.
Please help me understand what this ultimately looks like long term.
None of us knows, but we should be trying to build something great here.