The word "inevitable" is from the Longbet prediction itself

> "... In five years, it will be metered by the nanosecond to end users. This is not only predictable, but inevitable."

And that's the extent of the rationale for the prediction. The right thing to do when people insist their unfounded conclusions are "inevitable" is to take their money, but that's not an option here (the Long Now foundation keeps what they make on investing any "bet" for the duration, and the capital is paid to a charity of the winner's choice).