Just re-reading this, it's interesting to me that the first successful hard drive for the Mac (even Apple used them internally, and reluctantly waived the voided warranty policy if customers installed them), was essentially IDE based. Essentially, Apple rejected the third party solution, and went with SCSI (for better or worse), only to eventually move back to IDE less than a decade later. NIH (not invented here) policy at it's best.
I'd like to know if Jobs was involved with the development of the SCSI interface or not. Considering Apple rushed a non-standard version of it to Market 9 months after Jobs was removed as head of the Macintosh division, it could well have been a scramble after his departure. Based on what I've read about the re-design of the Macintosh case for the Plus by Frogdesign, it appears Jobs had little input on at least the appearance. Then again, it's hard to believe even Jobs could have ignored the success of the Hyperdrive ... So perhaps SCSI was the compromise, a faster connection port for an external drive, but still no internal drive or fan. Adding a fan would have been a simple matter for the Plus, as Pina's book describes ... And considering the problems the fanless Mac caused, one would think that would have been the first problem Apple engineers rectified after Job's departure ...