Hell, most people don't read FAQs either.
As for "can't" that's not accurate. You can certainly prevent old data from being pushed out, by returning an error on insertion of new data if there's no free space. That's how many other storage systems (e.g. every filesystem ever) work. While it's true that you can't fully protect against a reduction of capacity when nodes go offline, that's a very different issue. Freenet being cache-like rather than storage-like was a decision of convenience, not a technical necessity.
Freenet is designed for sharing information, not archiving it. You can't have both.