Windows Vista and Windows 7 happened to them, and Android/iOS tablets happened to their target demographics.
And notebooks got much lighter and smaller on average, compared to the netbook era. You used to need a 14" or 15" notebook with an inch worth of screen bezel to get a "full-powered" CPU and enough RAM to have a reasonable desktop experience, plus an unwieldy docking station to have enough ports to wire it up to a work place; netbooks were tempting companions to that.
Today you can get a full desktop experience (and even some fairly high end graphics capabilities) with a 13" laptop that's smaller than 11" subnotebooks and ligher than most netbooks used to be, and can fit a full keyboard.
E.g., a modern Asus PX13 is about the size of an 11" Asus eeePC, and just as heavy… but it comes with a 24-core Ryzen CPU, 32 GiB RAM, and an RTX 4070. If I'm on the move, I can just take that, instead of buying a companion device, and if I'm in the office, I can connect it to multiple daisy-chained monitors with connected peripherals using a single USB-C cable. Other Windows vendors have similar offerings.
And that's just looking at direct equivalents; completely ignoring the Apple-sized ARM elephant in the room. The M series CPUs are nuts in terms of performance, but the battery life is comparable to netbooks.
> Even the smallest Chromebook seems only slightly smaller than a laptop.
For a while companies made ChromeOS tablets to fill the gap between notebooks and Android tablets, but the higher end Android tablets these days have cannibalized that market too. And "higher end" is relative; netbook prices (400-ish dollars) get you fairly capable 10"-ish tablets from multiple vendors, complete with precision stylus and keyboard/touchpad cover. That's big enough to get netbook-tier keyboards (which were never great, let's be honest), but small enough to fit in cargo pants pockets (very handy for air travel), and the battery life is measured in days.
The closest things I've seen are the AlphaSmart Dana (discontinued), Apple eMate (discontinued) and the Freewrite line of devices.
I've thought about trying to make one myself from an Raspberry Pi, but those are not very low power and they seem to be tied to Linux (or at least I'm not aware of alternative operating systems).
I'm pretty sure the battery packs in both of them are just some 18650 in a trench coat, so at some point I'll probably attempt to replace them and hope I don't start a fire
The next best thing is e.g. the Surface Laptop Go with 12.4" screens. Any higher and you have e.g. the Macbook Air.
But as another commenter pointed out, a tablet with stand / keyboard is likely the best alternative nowadays.