Last week, Morgan Stanley’s Katy Huberty noted that Apple’s (AAPL) current stock price suggests that the market is expecting the company’s earnings to grow minus 2% in perpetuity. (See here.)

In the first of a two-part series, Bernstein’s Toni Sacconaghi on Monday drilled a little deeper into that -2% growth rate and found a series of what he calls “fantastically pessimistic assumptions.” Keying in on the iPhone, Apple’s single largest source of revenue, he wrote that for the company to grow -2% after 2012, either…

• The iPhone would have to lose three-quarters of its market share over the next three years, with sales falling from an estimated 116 million units in 2012 to 45 million in 2015
• Or Apple’s gross margins would have to drop more than 1,000 basis points, from an estimated 41.3% in 2012 to 30.9% in 2015, while gross margins on the iPhone fell 2,000 basis points

“What makes Apple’s valuation truly astonishing to us,” Sacconaghi writes, “is that the iPhone and iPad — which together drove 87% of its revenue growth and an estimated 91% of its profit growth last year — are exposed to secular tailwinds.”

In other words, the company is growing like gangbusters, not shrinking like its current P/E ratio would imply.