How many people read a version of the same story and pivoted their company to focus on SecondLife, NFTs, blockchain or whatever else technology was hyped at the time and tanked? That's the other half of this story.
- Virtual worlds / metaverses
You can replicate real life, but it's kind of boring.
- 3D printing
Became a useful industrial tool, but home 3D printing never went mainstream. At one point Office Depot offered 3D printing. No longer.
- Self-driving minibuses
Several startups built these, and some were deployed. Never really caught on. You'd think that airport parking shuttles and such would use these, but they don't.
- Small gas turbines
Power for cars, buses, trucks, backup power, and other things where you need tens to hundreds of kilowatts in a small package. All those things were built and worked. But the technology never became cheap. Aircraft APUs for large aircraft and the US Army's M1 tank variants remain one of the few deployed applications. The frustration of turbine engines is that below bizjet size, smaller units are not much cheaper.
- 3D TV
That got far enough that 3D TV sets were in stores. But they didn't sell.
- Nuclear power
Works, mostly, but isn't really cost-effective. Failures are very expensive and require evacuating sizable areas.
- Proof of correctness for programs
After forty years, it's still a clunky process.
- Maglev trains
Works, but insanely expensive.
- The Segway
Works, but scooters do the same job with less expense.
- 3D input devices
They used to be seen at trade shows, but it turns out that they don't make 3D input easier.
It's quite possible to guess wrong.