It's been an interesting year for Apple (NASDAQ:AAPL) -- by turns scary and exhilarating.

The iStuff maker surpassed ExxonMobil's (NYSE:XOM) market cap way back in January, and it's been the most valuable equity on the planet ever since. But the ride has been anything but smooth: Share prices peaked in September and have fallen more than 22% from those lofty all-time highs. The iPhone 5 and iPad Mini may be breaking sales records, but they have failed to wow analysts and investors.

Still, the stock has gained 32% year to date, leaving the Dow Jones Industrial Average (DJINDICES:^DJI) far behind. It's hard to complain about market-stomping annual gains like these:

AAPL Chart

AAPL data by YCharts.

Cupertino still dwarfs Exxon's $404 billion market cap by about 20%. Will Apple be the biggest kid on the block in 2013, too, or is it time for a change at the top? What if Apple never sees $700 per share again?

If you think the idea of Apple falling back is ridiculous, you haven't been paying attention. Giants stumble all the time, and even Exxon has been bested by several giants over the last decade. Check out this historical rollercoaster:

AAPL Market Cap Chart

AAPL Market Cap data by YCharts.

Microsoft (NASDAQ:MSFT) obviously peaked a few years before the start of this chart, during the overheated dot-com boom. The software giant has seen and squandered many chances to climb back on the throne, notably with the utter failure you know as Windows Vista. General Electric (NYSE:GE) suffered grievous damage in the financial crisis of 2008, being half a bank itself.

Grandpa Exxon will always rise and fall with the oil market. When we hit the dreaded "peak oil" plateau, the giant must adapt and start selling alternative energy solutions. Failing that, Exxon will shrink to a fraction of its current bulk.

Apple faces a similar innovation dilemma. The company has made an unparalleled fortune by extending and refining the basic iPhone concept over the last five years, but the market for these gadgets may soon reach saturation. And that's where Apple needs to adjust its business model, come up with mind-blowing new products (not just a bigger or smoother iPhone), or perhaps both. These are no simple "gimme" adjustments.