Furthermore, there is a level of replication such that even if nodes fail, the data will always be available in other nodes. 100% guarantee however is likely not possible.
100% isn't possible with centralized services either, almost by definition. Typical webhosts at best reach their advertised 99.9% uptime. Dropbox's servers could be hit by a nucular 'splosion.
This is just a natural circumstance as long as we live in a material world.
The best way to make sure the safety of your own data in such P2P systems like MaidSafe is just to safe-keep them as a local copy (as you probably would anyway). That way you'll still have them and you'll be the last remaining and perfectly valid node if you're the only survivor of an apocalypse.