- people finally discover how hard it is to hold on to their coins in face of determined hackers
(i.e. not the script kiddie level hacks we've seen so far), but stuff at the level of stuxnet
this is the only real threat
how hard is it to print out this page
https://www.bitaddress.org/ and send the coins their to the paper wallet?
That illustrates a weakness, not a strength.
Someone will host a similar page that sends the private key back to the server. Then they send out 100 million emails telling people to "click here to make their bitcoins safe with a paper wallet".
The biggest contribution of Bitcoin to society may not be the monetary improvement that it brings; it may be the improvements that it forces to be developed for computer security.