I think it's similar to publishing a Torrent. You publish it to the network, it does stay at a particular place. Mirroring is common and that's what will achieve decentralization of Nostr.

At some point people will run their own relay and it will mirror notes they are interested in for easy access and archival. And maybe provide read access to others.

I run my relay and mirror many things with a simple shell script from cron.

The rule is - if a client can get it, so can a relay, which can act as a client. 

I own my data in a sense that I own my identity and that I have the archive of it that no one else can delete. 

But as Peter said, it's digital, can be copied and people have signed it with their keys. Internet does not forget.