But I'm not entirely convinced that IBM just didn't know how to do this. E.g., there's the Aquarius concept (1977), which had progressed to working production prototypes, which would have come attractively packaged and with an app store based on bubble memory cards. (Apparently, this was canceled over concerns regarding the reliability of those bubble memory cards.) As a marketing concept, this would have been about 25–30 years ahead of its time – and it would have separated this neatly from any other IBM business. However, any such elaborate concept would probably have struggled in an organisation like this, where any move may endanger what has already been secured.

Maybe, the remarkable lack of context of the IBM 5150 was its internal selling point?

(Imaginary internal sales pitch: "See, this machine has no specs. We won't even say what it's for. We'll just tell them, the Little Tramp likes it, so you like it, too. No, it won't eat into mainframes.")

*) More about the IBM Aquarius (including photos) can be found in Paul Atkinson's book "Delete. A Design History of Computer Vapourware", Bloomsbury, 2013.