In September 2015, Apple (NASDAQ:AAPL) introduced the iPhone 6s. The device included several big improvements over its predecessor, the iPhone 6, such as a way faster processor, improved camera subsystems, second-generation Touch ID, and a pressure-sensitive screen enabling the company's 3D Touch functionality.

3D Touch was widely viewed, particularly ahead of the formal launch of the device, as the signature feature of the iPhone 6s.

Apple's iPad Pro tablets.

Image source: Apple.

However, the iPhone 6s family failed to capture customers' imaginations, and 3D Touch, though innovative, didn't usher in the revolution in human-smartphone interaction that Apple may have hoped for.

Since then, 3D Touch has been included in each of Apple's subsequent iPhone generations. The iPhone 7, iPhone 8, and iPhone X (pronounced "iPhone ten") all continue to implement 3D Touch.

Where 3D Touch is conspicuously missing, though, is in Apple's iPad Pro lineup. The first 12.9-inch iPad Pro, introduced in late 2015, doesn't include the technology, nor did the 9.7-inch iPad Pro released in March 2016.

It's common for a new technology unique to either iPhone or iPad to take a generation or two to be included in the product categories they weren't originally introduced in. However, Apple's recently released second-generation iPad Pro tablets, launched earlier this year still don't incorporate the 3D Touch technology that made its debut two years ago.

A question worth asking, then, is: Will 3D Touch ever come to a future Apple iPad?

The answer isn't clear

Had 3D Touch turned out to be a rousing success that became an indispensable feature in Apple's smartphones, then it would be a no-brainer for Apple to bring the technology to the iPad. After all, considering Apple needs to work incredibly hard to persuade iPad users to upgrade to new models, paradigm-shifting technologies -- if 3D Touch were one -- could certainly help Apple in that regard.

However, it doesn't look as if 3D Touch really took off as Apple had hoped. Yes, it's a nice feature in the current iPhones, and yes, whenever I use phones that don't incorporate 3D Touch, something seems "off." But it's clearly not a game-changer.

It appears implementing 3D Touch on large-screen displays like the ones found on the iPad Pro tablets is much more difficult than implementing it on a smaller iPhone-sized displays.

Apple has the resources to make it happen, but I believe the engineering effort to make it work would be substantial, and the cost added to each new iPad sold with the technology would be more than trivial.

Though Apple sells premium products, it can't afford to add expensive features into its products that it doesn't think will help boost its business. Remember that manufacturing cost increases can lead to lower margins, product selling price increases (which could crimp demand), or sacrifices elsewhere in the product to stay within budget.

If Apple believes that adding 3D Touch would help catalyze iPad upgrades or bolster its competitive positioning in the premium tablet market, where it is virtually uncontested, then it'll happen. But if Apple thinks adding it would just be a significant engineering exercise and a cost addition that won't really get people to buy new iPads, then it'll remain an iPhone-exclusive feature. I believe the evidence is there to suggest that's what will happen, given the lukewarm response to the technology on the iPhone.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.