It's still true that people, out of convenience and familiarity, used Raspberry Pi for tasks where a microcontrollers would have been perfectly adequate

There was definitely usecase overlap due to the presence of the GPIO, but huge numbers of Pis ended up doing things a microcontroller can't - stuff like the PiHole and Retropie projects, and never used their GPIO pins at all.

Thinking of any of the early Pis as microcontrollers ignores a huge amount of the ways in which actual end users interacted with the thing, and even the way it was sold and marketed. Upton was trying to replace early hacker-friendly home computers like the BBC Micro/Apple II, for a new generation.

This just clarified something for me. I've always been annoyed when I see a Pi with nothing connected to its GPIO header; why not just use a cheap thin client? Or an old laptop, for that matter? But that's missing the point. Here's the point:

Pre-Beagleboard-and-Pi, if you wanted an programmable thing to work with GPIO, you used an Arduino or a BASIC Stamp, or just a plain old PIC. But they wouldn't run a real OS.

Pre-Beagleboard-and-Pi, if you wanted an embedded Linux box, you used a WRT54G or a Soekris or an old laptop. But getting GPIO out of them was a PITA. (And often involved lashing an Arduino to the side.)

The Beagleboard (released in 2008), could finally do both. It had gobs of I/O and first-class support for it under Linux. It was pretty affordable. Then the Raspberry Pi came out in 2012, with a similar amount of GPIO, but demolished the price point to where it made sense to use it in place of a microcontroller.

That's really the magic of the Pi. You can keep one cheap gizmo around, and use it to solve (a large fraction of) two classes of problems. It doesn't fully replace everything a PIC or a PC can do, but it replaces an awful, awful lot of them.