I would argue that the whole "IBM PC clone" phenomenon came about more by chance than by design: IBM certainly didn't want it to happen, but since they cobbled their PC together from standard off-the-shelf parts, and outsourced the OS to a third party (Microsoft) who was more than happy to license it to others too, there wasn't much they could do to stop it either. The other platforms you mentioned were ultimately undercut by these cheap PC clones and by their own failings, not necessarily by the fact that they were proprietary. Actually, IMHO Apple only narrowly escaped joining the Atari ST and Amiga in the mid 1990s...