"Mainframes, minicomputers, desktop personal computers, laptops, and now tablets and smartphones" -- yes, so we had like 5 generations of computing hardware platforms. And we began with so premature and begging for improvement technologies (when they started in the 50s) and so large form factors that we had Moore's law going on for decades until a few years past.
Now we've reaching physical limits in CPU shrinking, and we have components so small (e.g. Apple Watch) than if they were any smaller you couldn't handle their buttons, or fit a camera lens, or speakers, or a large enough screen in there.
My point is, just because something has been going on for 5 generations of form factors, it's nowhere near enough evidence to be meaningful statistically or otherwise to deduce it will go on forever.
A turkey is fed every day by the farmer for months on end. But after 300+ observations that make it think feeding is never ending and inevitable there comes thanksgiving.