You are contradicting yourself. On one hand, you state that a "killer app", i.e. something that everyone would use is cryptography. On the other hand, you state that the vast majority of people trust a squiggle on paper, rather than actual crypto. If the average joe doesn't care about crypto, why would a crypto email be interesting?
You'd need to find a way to make crypto interesting. Lots of email don't have crypto, so if you sell something that does crypto well, then you can corner the newly created crypto market.
A peer-to-peer system for sharing music/films that does good strong crypto (and faster than tor) would do the job.
It's baffling to me that a squiggle on a bit of paper is more trustworthy than a properly implemented cryptographic signature.