That finding follows more than a year of declines in tablet shipments and sales, due partly to the availability of larger smartphones, often called phablets, like the Apple iPhone 6 Plus and 6S Plus with a display size of 5.5-in. or the Samsung Galaxy S6 Edge Plus or Galaxy Note 5 at 5.7-in.
Gartner analyst Meike Escherich also named free tablet operating system upgrades as another reason that people hold onto older tablets, rather than buying a new one.
"Most applications work pretty well with first- and second-generation tablet hardware and because the operating system can be upgraded for free, the user is not compelled to change the device," Escherich said in a statement.
In the U.S., more than two-thirds of households already have a tablet, and one-fourth of households have two or more tablets. Escherich said that unless new compelling innovations or incentives to upgrade are created by manufacturers, the installed tablet market will continue to fall.
In the worst-case forecast, many tablet users will never upgrade, as phablets and two-in-one convertible PCs can readily take the place of a tablet, Escherich added. In markets like the U.S., it is possible that household penetration for tablets could fall under 40%, she said.
With a larger smartphone, or phablet (5.5-in. or larger), consumers find little benefit in owning a 7-in. tablet that often lacks phone capability. This applies especially to budget-minded buyers who prefer owning a single device instead of two.
The survey also found that 44% of current tablet users planned to substitute their tablet with a different device.
Gartner's survey was conducted in May and June with 19,000 consumers in the U.S., UK, France, China, Brazil and India.
Earlier this year, Gartner said tablet shipments would decline by 13% in 2015 over 2014, reaching 192 million tablets shipped globally.
Last week, analyst firm Canalys said tablet shipments globally fell by 20% in the third quarter of 2015, compared to the same quarter in 2014. Apple saw a decline in iPad shipments in the quarter of 20%, falling below 10 million tablets for the first time since 2011, down to 9.9 million tablets.
Samsung shipped 7.9 million tablets in the third quarter, about a 20% decline as well, Canalys said. In third place, Lenovo actually saw a 2% increase to 3.1 million tablets shipped. Greater China saw the steepest decline, falling about 34% for the quarter.
Canalys predicted that Apple's launch of the iPad Pro and iPad mini 4 will boost shipments in the fourth quarter of 2015, although the biggest impact will come from an expected launch of the iPad Air 2.