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"TBH I'm amazed there aren't knock-off Pi-es out there. Is there anything proprietary or custom to them?"

There are plenty of similar products. Many of them are so obviously meant to be like the raspberry pi that they've called themselves "[insert other name of fruit] pi". Some actually have better specs than the raspberry pi. However, they are less popular for many reasons. One reason is that they don't have as much community support, sometimes supporting fewer operating systems or working in an incompatible way. Another reason is that, while they often support similar hardware modifications, they aren't directly compatible with the ones designed for the raspberry pi. A third reason is that these usually don't have a guarantee of continued manufacture or software support, something all pi models have received since they first came out. But if you're looking for other versions, you'll find them thick on the ground.

"They are verging on being usable low-end computers now."

They passed this point for many users quite a while ago. It depends what you're doing on them, but for traditional office tasks, the version 3 was quite capable of the load. If you need a lot of memory, nothing before the pi 4 gave you more than a gigabyte, but plenty of use cases didn't need that. It won't replace the full desktop for the people who need that amount of performance, but it could probably replace many an old one.

"Perhaps an official PiBook (in the vein of the OLPC XO netbooks) might be in the offing?"

The PiTop people did make one of these. I'm hoping some other people will also do so, as I found their one somewhat overpriced and underwhelming. Unfortunately, for the price of their enclosure, you can get a comparable laptop with better battery life, builtin storage, and a slightly faster processor. I'm hoping that people will start to realize the potential of using the pi as the computer for various form factors.