Great question. IPFS doesn't make any guarantees regarding durability, because ultimately this depends on people caring about the file, either because it's valuable to them, or they're deriving indirect value (being paid to care).
People that care about the data's presence will pin objects locally:
- I pin + seed my own files, or files I'm interested in keeping alive.
- I can hire a service (or multiple) to pin + seed my files for me.
- I can use things like http://filecoin.io to incentivize large groups of people to seed files for me. (this is why Filecoin + IPFS are sister protocols)