CHAPEL HILL, N.C. (MarketWatch) — Amazing as Amazon.com’s stock has been this year, it will have to be even more amazing to live up to Wall Street’s expectations.

That's worth remembering as investors digest the initial reports about retail sales on Black Friday and Cyber Monday — reports that otherwise might prompt you to jump on board the Amazon AMZN, -4.12% bandwagon. Those reports show — just as they have for many years now — that Internet retailers such as Amazon are taking a big chunk of market share from brick and mortar retailers.

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So far this year, Amazon’s is the second-best performing stock in the S&P 500 SPX, -2.37% , with a gain of 117%. Only Netflix NFLX, -4.18% has performed better

But to what extent will Amazon continue to be able to capture other retailers’ market share? Extrapolating Amazon’s past growth rate into the future is a risky business, and none of the top-performing investment advisers monitored by the Hulbert Financial Digest appears willing to do so.

To quote John Maynard Keynes, the famous British economist of the early 20th century, “Trees don’t grow to the sky.”

To appreciate how fast Amazon must grow to satisfy Wall Street’s expectations, consider the year-over-year increase in its annual sales. Since 2011, this annual increase has been quite consistent, ranging from a low of $13.0 billion to a high of $14.5 billion — averaging $13.7 billion per year.

Between 2016 and 2019, in contrast, according to FactSet, the Wall Street consensus is that Amazon’s sales will grow by an average of $25.9 billion per year, or nearly double its past-four-year average. (See chart above.)

Amazon’s true believers are not fazed by these numbers, since they amount to a “mere” extrapolation of the company’s growth rate into the indefinite future. After all, as the company gets bigger, the same percentage growth rate translates to a bigger sales number.

But Amazon’s size cuts both ways. As the law of large numbers teaches us, a company will find it increasingly difficult to maintain its growth rate as it gets bigger. Why should Amazon be exempt from this law?

Nejat Seyhun, a finance professor at the University of Michigan, says there’s another factor that Amazon’s investors appear to be overlooking: The vigor with which other retailers will fight back to keep Amazon from grabbing as much market share as Wall Street expects.

A decade ago, of course, many of those other retailers were ill-prepared to exploit the potentials of Internet retailing. The $13.7 billion per year of sales by which Amazon increased its market share represented the low-lying fruit on those other retailers’ trees.

Amazon’s days of easy picking could be mostly over, since those other retailers are today better prepared to exploit Internet commerce. This may make it difficult for Amazon to continue taking even $13.7 billion of annual market share from the other retailers, much less the $25.9 billion that Wall Street is forecasting for 2016 through 2019.

Amazon’s stock price would otherwise be in a better position to withstand a diminishing sales growth rates, if its price/earnings ratio weren’t so high. But it instead is in the stratosphere — near 1,000-to-1 currently — a level that Prof. Seyhun calls “crazy.”

“ Investors looking to bet on the high-tech and Internet sectors may want to favor Apple over Amazon. ”

These are some of the reasons why the top-performing investment advisers I monitor don’t detect much value in Amazon at current prices. I define this group of top performers as all those on the Hulbert Financial Digest’s monitored list who have beaten a buy-and-hold in the stock market over the last 15 years—a group that currently contains 43 advisers. This 15-year period is long enough to largely eliminate the role of luck.

The stock that these top performers like the most right now is Apple AAPL, -4.19% , which is recommended for purchase by 11 of the 43 top performers. Investors looking to bet on the high-tech and Internet sectors may therefore want to favor Apple over Amazon.

Tied for second-most popular among the top performers are four more stocks, each of which is recommended by seven of the 43 top performers: AT&T T, -2.24% , Disney DIS, -3.08% , Johnson & Johnson JNJ, +0.15% , and brick-and- mortar retailer Target TGT, -1.99% .

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