Apple is going to have a tough year

The moment it becomes more cost-efficient to buy a bunch of some new components early, Apple does so—but then the cost of having them sitting around doing nothing when they could be put to use steadily rises. Meanwhile, it can also become increasingly expensive to keep ordering "old" components for your existing design, because the rest of the industry has moved on from using that component and you're the only one the suppliers are still producing it for, and they really want you off it too so they can shut that line down. Together, these effects imply that it actually becomes cost-effective from an operational logistics perspective to refresh the iPhone's hardware each year, even if the "design plan" is written in terms of two-year milestone releases. Whatever ends up being cost-effective to put into the interim refresh gets advertised on stage, but—unlike the actual "planned" releases—it's effectively random, rather than a "curated" list of features that go together to form a whole.

Or, to put it another way—Apple likely went to the drawing board and designed the iPhone 7 right after the release of the iPhone 6. Some random subset of the components of the iPhone 7 ended up "resolving" early enough that they got "binned" together as the iPhone 6S. Whatever components didn't become available in time for the 6S release, are now waiting until the 7.

When you see a number-increment release like the 6 or the soon-to-be 7, imagine that all the features that went into the previous "S" release were also announced on stage at the same time that they announced whatever did go into it. Because that's what Apple intends to happen; together, the 6S's features and the 7's features (or the 5S's features and the 6's features, etc.) add up to a coherent and compelling product vision. The "S" release just takes a random bite out of that vision and sputters it out early, leaving the actual numbered release to say whatever's left over.

Not necessarily. A stock represents the market's valuation of a company, including its future earnings.

Maybe a plateau to growth is already factored into the market's valuation of Apple (arguably, you'd have to be an idiot to think that Apple's cash cow will grow forever and magically never reach saturation). The stock's value could continue to hold steady and even grow while the company's revenue declines if that decline is less than expected.

They could merely decline/plateau at a slower rate than the market has priced it at. Or they could use their gobs of cash to continue the buyback program.

In fact, Apple's buyback program is expected to reach $200 billion by 2017. Even if Apple's revenue stopped growing tomorrow, they would throw off enough money to run a huge buyback program for years to come.

My own prediction for Apple in 2016 is that the iPad returns to growth this (calendar) year.