With ever changing world events, any prediction beyond short term, 6 - 12 months, is a shot in the dark IMO

They can't even get that right. Look at how they've been predicting a shortage that didn't materialize last year and might not this year either. I just read that next year there are a number of significant contract renewals in Chile and that might interrupt supply.

However, they do know the output of all the mines, and they know of the ones coming on stream so they can make rough predictions. They also know that a percentage of unanticipated events occur every year to disrupt supply, whether natural disasters or labour disputes or bad financing.

They know the potential for the largest supply and work downwards from there. There just isn't enough supply coming, regardless of any supply disruptions.

Given this rough estimate everyone seems to be anticipating a major shortfall in about four years. I remember Don Lindsay saying this past year that 2017 was going to be a great year.