This is a common expression and it's bound to be true. IMO, it means nothing. 20 years is too long, there is adequate time for bitcoin to be so successful that we moved on to another generation of crypto currency. So transaction volume would still be zero in 2030 whether Bitcoin is hit or not with the 20 years. 

I believe Satoshi knew he was on to something, but he wasn't sure how big or how fast Bitcoin can grow. So he picked a nice fail-safe number of 20 years. 5 years on, now in 2015, can we be any more sure of a very large volume in 2030?