A couple of thoughts:
- The cryptography could be broken. Nobody would know about it.
- The block chain gets too big to maintain for the average bitcoin user. We would know about it and possibly be able to fix it in time.
Terrible, terrible thoughts.
Really fighting the urge to rant here about how dumb this is.
It gets broken? Change algorithm.
Block chain gets too big? Only make users download the newest part of it (up to a year?), or mini blockchain.