"I am convinced that cross-vendor incompatibility was THE reason for CP/M's failure. Not anti-competitive behavior, not shenanigans, but the fact that if you spent $495 on the Kaypro version of Wordstar and then bought an Osborne, it wouldn't work. Same Z80, same CP/M, wouldn't work."

But CP/M had a well defined API. Compliant programs would work on different vendor's computer, much like in the MS-DOS domain. The key difference was, that CP/M had no well-defined disk format, i.e. you couldn't just swap disks (for evaluation purpose only, of course) with your buddy if he didn't happen to own the same type of computer; you had first to transfer the software via other means, e.g. serial interface (perhaps using kermit). A bridge too far for most casual computer users.