Spot on, except....

2015? You can't assume capacity will double every year. Assume 18 months and you'll be a lot closer. so, make it 2017-2018.

Oh, and you'll still need to actually back the things up in case they get stolen or burned. Except there will be LESS need to back up as they aren't as likely to just die... so people will back up even LESS. Unless it's built in + automatic. No one backs up there personal data nowadays anyway really and even less than no one will in future (no one = < 1% of the population).