Apple Inc. (AAPL) is one of the most popular and most speculated-on companies out there. And after a big pop after earnings, the tech giant has once again become the talk of Wall Street.

So allow me to add my voice to the unwashed masses, irresponsibly speculating about Apple stock and whether it will go up or down from here.

I think it’s going down — and AAPL stock may continue to struggle for some time.

Now, there’s no shortage of “experts” out there with their cute or outlandish takes on Apple stock, looking for 15 more minutes of fame. Just consider that the highest price target right now is $777 from Cantor Fitzgerald analyst Brian White — the same guy with previously cutesy targets of $1,111 and $888 over the past few years for AAPL stock — while the low price target is a mere $270 per share from Ed Zabitsky, a one-man research firm who has stubbornly stuck to his low target since 2012.

It’s admittedly hard to figure out which analysts are honestly giving their take and which ones are simply trolling investors and CNBC anchors to get their ugly mug out there.

But I think if you take a logical look at the company and its numbers, you can carve through the hysterics and get right to the heart of Apple’s growth challenge in the coming months and years.

To me, that challenge begins and ends with the iPad — and I expect continued disappointment in Apple’s tablet sales to act as an anchor on AAPL stock. Here’s why:

iPad Sales Were Ugly as of the Most Recent Apple Earnings Report

Last quarter, “weak” iPhone sales overshadowed an otherwise strong earnings report for Apple stock. This time, investors were pleased as the smartphone bounced back with a forecast-topping 43.7 million iPhones sold during the quarter, beating analyst estimates by more than 5 million units.

But while the iPhone remains the biggest present driver of AAPL stock, with the device generating $26 billion of the $45.6 billion in total sales on the quarter for a huge 57% share of the top line, the iPad is the future of Apple … and iPad sales aren’t looking good.

Consensus estimates were for around 19.7 million iPads to be sold last quarter, up fractionally from the 19.5 million sold last year. But Apple sold just 16.35 million iPads for a 16%-plus decline in iPad sales.

This is ugly given the fact that the broader industry trends still are very much in favor of growth. Even as the tablet market matures, research firm IDC still predicts a nearly 20% jump in device sales this year over last.

The fact that Apple can’t even tap into the organic growth of this segment shows the big risk it faces from competitors in both the short term and as AAPL stock relies on its product upgrade cycle to juice numbers via its loyal customer base.

The iPad Is the Future of Apple

A host of tech experts have been talking for some time about why tablets will ultimately supplant smartphones for many uses.

Consider that back in 2012, Keven Tofel of GigaOm wrote that tablets will replace the smartphone for a variety of reasons — the rise of video content, the ability to use headsets to make calls of similar quality, the evolution of user interfaces and others.

I have my doubts about whether tablets will fully replace smartphones, particularly in fast-growing emerging markets. The fact that Gartner reported China’s smartphone sales soared 86.3% in 2013 is proof that the smartphone is alive and well.

However, we’re not talking about all smartphones here. We’re talking about the iPhone … and the future isn’t quite so growthy.

While Apple did sell 20 million more units in 2013 than in 2012, its market share of global smartphone sales slipped from a 19.1% share to 15.6% year-over-year, according to Gartner.

Besides, as competitors keep offering high-powered phones that are just as good — or, at least, good enough — Apple is forced to move downmarket with its smartphones at the cost of margins.

If the tablet market broadly is flagging, that’s a concern. But consider that recent estimates show that the iPad is falling even faster in tablet market share than the iPhone — just 19.1% share, down from 40% a year ago.

Bottom Line

Apple needs to claw back market share in a hurry. And if it’s fighting against a slowing tablet market where competitors are slashing prices and just as desperate for tablet sales, it’s going to be a rough road for iPad sales in 2014 and beyond.

This could create a big stumbling block for AAPL stock as the company looks to innovate and expand its product line.

Because if all Apple winds up being is an iPhone company that sells a small number of tablets … well, AAPL stock isn’t going to be all that attractive to many investors.

Jeff Reeves is the editor of InvestorPlace.com and the author of The Frugal Investor’s Guide to Finding Great Stocks. Write him at editor@investorplace.com or follow him on Twitter via @JeffReevesIP. As of this writing, he did not hold a position in any of the aforementioned securities.