Spinning storage has at least 10 years life (growth) with 'todays' technology before the dreaded super paramagnetic limits are hit.

The question is...Will we need the capacities offered by HDD for red book CDs when solid state becomes cheap at say 10Gb?

Storage needs are always driven by user needs - if red book stays the format of choice, the capacity per CD will stay the same - thus the need for storage will hit critical mass at a point where we simply dont need any more storage (ie 500 CDs worth of storage for a £50 solid state stick will obliterate the need for HDD)!