The worst is yet to come for Apple's (AAPL -1.22%) iPad. Windows tablets, in contrast, have a bright future.

That's according to a recent report from IDC. The research firm expects shipments of Apple's tablets to decline over the next five years, while tablets powered by Microsoft's (MSFT -1.27%) Windows operating system more than triple in popularity.

No speed bump here
Apple's CEO, Tim Cook, has described the iPad's current problems as a "speed bump." iPad sales have been in decline for more than a year -- down 18% on an annual basis just last quarter -- but Cook believes, over the "long arc of time," the iPad will recover and sales will blossom.

IDC disagrees. In 2014, Apple shipped 63.4 million tablets -- 27.6% of the market. IDC believes this figure will contract to 60.1 million this year, before recovering somewhat, but believes Apple will ship only 61.9 million tablets in 2019, giving it a theoretical market share of just 23%.

If IDC's projections prove accurate, Apple will sharply underperform the tablet market. Over the next five years, IDC expects tablet shipments to grow more than 17% -- but believes Apple's tablet shipments will decline by about 2%.

Apple can certainly survive without the iPad -- its profits have climbed to record highs even as the iPad has struggled -- but it may need the tablet for reasons of diversification. With so much of its revenue coming from just one product (iPhone), any disruption in the smartphone market could take a significant toll on Apple's earnings.

A larger niche
In contrast, IDC is quite bullish on Windows tablets. The research firm believes Microsoft's next operating system, Windows 10, will prove to be a boon for the platform, and juice sales of Windows tablets in the quarters to come.

IDC does not believe that Windows tablets will crush the iPad -- in fact, it still expects the iPad to outsell all Windows-powered tablets by a healthy margin in 2019 -- but believes that Windows tablets will strongly outperform the broader tablet market in the years ahead. Last year, Windows-powered tablets accounted for 5.1% of the market -- 11.6 million shipments in total. IDC believes this number will surge by nearly 230% to 38 million in 2019.

That would be fantastic for Microsoft, whose Windows business has come under pressure as sales of traditional PCs remain depressed. It has seen a modest level of success with its Surface Pro 3, but Microsoft is basically a non-factor when it comes to tablets -- 15% market share would at least represent a solid base.

It's all up in the air
Still, I'm quite skeptical of IDC's predictions. Consider the firm's tablet projections back in 2012.

Those figures look downright silly today. Apple's iPad would have to undergo quite a renaissance to hit 50% market share next year. Windows tablets would have to double in popularity, and demand for Android tablets would have to fall off a cliff.

The tablet market remains dynamic -- Windows 10 offers a number of features that should make it compelling to tablet buyers later this year, and in the years to come, but Apple's deal with IBM and its widely rumored iPad Pro could shake things up. Windows tablets' greatest appeal is like to be to enterprise customers -- if Apple can get the iPad to catch on with business users, these projections could look just as terrible five years from now.

IDC is a well-respected firm, and its projections are definitely worth keeping in mind, but I would urge investors to take them with a healthy grain of salt.