Apple's gross margins on iPhone sales — a measure of the raw underlying profitability of its business — have declined from 57.7% in 2009 to just 41% today, according to analysts at Bernstein Research see chart below .

They expect it to sink to a "mere" 39% in 2018.Apple is the single most profitable company on the planet.

So it is unusual to see analysts worrying about where future profits might come from.To put this in context: Apple is going to stay very, very profitable.

No one is saying those profits are going to disappear.

But at least four teams of analysts said in recent notes to clients that it was becoming harder to see how Apple would sustain its massive gross margins.Here's the chart from Bernstein's Toni Sacconaghi et al: Key issues include the cost of making next year's iPhone and whether Apple still has the ability to raise prices even further despite price competition.

"Historically, iPhone gross margins have deteriorated steadily, and we believe margin compression underpins the bear thesis on the stock.

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