It's a category on the decline. This is not a fallacy this is a fact. Here are some actual facts to add to the discussion...
Last three years in unit sales
2016: Tablets: 174.9 million, PC (desktop/laptop): 260.1 million
2017: Tablets: 163 million PC (desktop/laptop): 260.1 million
2018: Tablets: 150.3 million PC (desktop/laptop) 256.7 million
iPad specifically..
iPad 2016: 45.59 million
iPad 2017: 43.75 million
iPad 2018: 43.5 million
Bear in mind that at its peak in 2014 Apple were selling nearly 68 million a year.
Sooooo....
Over the 3 years you listed tablets overall dropped 25 million (15%) but the iPad only dropped 2 million (5%) and somehow that's bad for Apple? I know Apple stopped reporting sales figures, but iPad revenues have been up the last 3 quarters compared to a year ago. You know, if you want to look at more recent data.
Its still a decent business for them, based on those numbers iPads account for nearly a third of all tablet sales.
The industry in general seems to be turning away from tablets though, Google are done with them and Microsoft seem to have backed away from the Suface Pro "Tablet with a keyboard cover" device a bit recently. The Surface Book and obviously the Surface Laptop suggest a move back towards traditional laptop style hardware.
Hardly. The market is turning away from tablets other than the iPad. Developers are still cranking out new and powerful Apps for the iPad (including full blown Photoshop) and Apple continues to push the envelope with their 'X' versions of their SoCs. Now with iPadOS Apple is making the iPad even more useful than it was.
Microsoft is moving back because they realized that Windows Apps that aren't properly optimized for touch aren't that great to use. They had the same problem Android did - developers who can't be bothered to optimize for tablets, and take the easy way out relying on built-in tools to make Apps "just work" on tablets. Except they don't.
Google doesn't seem to know what they want to do. Google Chrome, running Android Apps on Chrome (which is pointless as developers who won't optimize for tablets certainly won't optimize for Chromebooks), Fucshia....they don't seem to have any kind of plan regarding their various OSes.