They have always lived and died by one product. First it was the Mac, then the iPod, now the iPhone. At the current scale, yes, the iPhone sustains 70% of the company. If the iPhone fails maybe the Mac will be the one product to sustain a smaller Apple. Maybe the iPad will rally and do the job.

It's not that weird of a predicament. Not all big companies are diversified. Microsoft has sustained itself for decades with only two products (Windows + office). Google is sustained basically by advertisement on search results. They have other revenue lines but not as important.

If these lines fall companies do the same any other complex organisms do. They adapt to survive, find new businesses, reinvigorate old ones... Or they die. It's really not that exceptional.

What makes Apple exceptional is the scale of it all.