Originally, Carbon was not in the plans at all (the yellow box corresponded to what is nowadays referred to as Cocoa). Adding that was a substantial additional task, but probably well worth it.
And the "yellow box" stuff had already been ported to run on top of Windows NT, which probably suggested those layers were pretty loosely coupled to the rest of the NeXT OS, and might be easier to integrate with Mac OS or whatever they came up with.
On this thought I left out the classic runtime because it was needed with either direction, my point was many at the time saw BeOS as the shorter path to a modern OS. Because NeXT needed and got a reworking from the ground up. It is now apparent that that was the plan all along but from the arm chair quarterbacks, back then it seemed like a foolish move, a fair percentage of Mac users back then where rooting for Be because it had almost mystical quality for the time, it was like Amiga in it's day. I am not saying Apple made the wrong decision, just that back then most people including myself could not understand the decision. If you can find any of the old Mac User or Macworld rags from that period there where articles and articles lamenting over how they could not choose such an advanced OS as Be.
http://lowendmac.com/musings/boxes.shtml
Originally, Carbon was not in the plans at all (the yellow box corresponded to what is nowadays referred to as Cocoa). Adding that was a substantial additional task, but probably well worth it.