The original motivation for the Pi was to offer current generations the same learning opportunities that low cost home computers offered those growing up in the 1980s. An expensive shared family PC in the 2000s didn't encourage tinkering. And things would get even worse as these PCs got replaced with closed smartphones as the only computing platforms people would grow up with.

So a really cheap computer, even if limited, would allow the young programmers to experiment and even if they ended up damaging it at a sufficiently low cost it wouldn't be a disaster.

The initial prototype hooked up two of the same processor in the first Arduinos, one running a Python interpreter and one generating a TV output. Booting to a programming prompt like the 1980s micro would encourage learning the language.

The second prototype used a much more powerful chip and the third added an ARM that could run Linux (it was the one shown in 2011 which looked like a fat USB pen drive). They realized that though the 1980s home computers booted to a Basic prompt, most of the time they loaded ready made games from cassette tapes instead. Having Linux would open up the modern Internet to the little machine.

I think they could have had it both ways - with a small local flash they could boot to a Python prompt if the SD cards was not present. With the card you would go into Linux. In a way, the original idea is finally here with the RP2040 microcontroller configured for MicroPython even though that has the flavor of cross-development instead of a stand-alone machine.