This sounds like the kind of argument people made when the iPhone had a software keyboard and every other smartphone had a physical tactile keyboard.
What I think you're missing is that the world will evolve in ways to accommodate a mouse-light interface or evolve to include it if it becomes obvious that it's mission-critical (it's not at all obvious today, despite legacy thinking). Neither version of the future precludes the iPad at all. Just look at the new gestural interface on the new iPhone X/XR/XS devices. It makes navigating the OS effortless and super fast in ways that it simply wasn't before. Before it, people were saying that it's too slow to navigate on a mobile device. How many other innovations like that are coming down the line? We don't know.
I say this as someone who's used an iPad Pro along with the Keyboard Cover for quite some time. No, you won't be able to type on it for hours and hours. But you'll be able to type on it for an hour or so consistently. That's a BIG deal, and changes A LOT. It's only going to get better from here. There are a lot of apps that support countless keyboard shortcuts, including Google's whole G Suite, Office 365, Lightroom, and the upcoming full version of Photoshop in 2019.
If you asked me 3 years ago if I thought the iPad would replace Laptops, I'd tell you probably not. Apple had done little to differentiate it from the iPhone lineup in terms of iOS or even hardware features. But with the iPad Pro specifically, I think things have taken a real turn and Apple is signaling that they're ready to focus on taking it from a media consumption device to a more productive, mission-critical device.
120hz refresh rate, full digitizer, Apple Pencil, the new integrated keyboard cases, a new focus on pro apps, and a ton of improvements to the actual UI/UX to make multitasking and app-switching easier....etc etc. These were all the things that were missing for a long time. But Apple is finally doing all the things needed to move iPad in a direction where people will choose it over laptops at an increasing rate.
The kicker is this: Apple announced recently during WWDC that iOS apps will work on MacOS. Many of their new stock apps in MacOS Mojave are just ported iOS apps. They are truly and confidently doubling down on iOS and iPad.
That's my personal prediction and belief anyway.