You see what you want I suppose. Even a small gain is a gain -- there 's nothing wrong with steady either. And I really don't know where these numbers are coming from or how accurate they are; certainly the Apple Watch numbers are ALL guesswork. Again, I'd like to see how it compares against individual MacBook products, or individual iPads -- it would be interesting indeed to see if a particular model had a similar curve. And the Macs in general would likely have similar performance, and sales figures for individual models -- yet nobody's calling them an abysmal failure.
Besides, all of this fails to take into account the fact that the iPad set the standard for tablets, and nobody, even relatively good Android devices, have been able to catch up -- even Android phone users likely own iPads. Comparing the Surface to the success of the iPad, and suggesting nobody wants it, or its features, based on the relative success discounts the fact that it is currently selling steadily year after year, despite its flaws, specifically as a tablet. Suggesting that is a primary reason not to add a trackpad to the iPad is at least flawed -- if that's indeed your suggestion.
iPad sales are dropping during the same period the Surface is rising and holding steady or edging up. Is it just a coincidence that Apple has started to market the iPad more like a Surface-type device? Maybe. I'm certainly not suggesting Apple compete directly with the Surface, as I have no great desire to see a touch interface added to the macOS (despite the fact they've added one in the form of the much maligned Touch Bar), but I certainly wouldn't balk at carrying a single tablet device that could instantly switch from iOS into service as a Mac simply by adding a keyboard and trackpad. And I would definitely like to see the same tools made available to me when attaching an external keyboard and display to an iPad, namely the ability to control the device with a remote pointer; and if nothing else, at least the text cursor to increase ergonomic workflow.