Er... Doom was not ported to the Amiga til 1998 (was it?), so by then, John Carmack would obviously have not cared about making an Amiga source port, since it wouldn't have been profitable. Calling him a bad programmer has to be the most idiotic thing I've ever heard of. The man was responsible for programming the game engines for Doom and Wolfenstein 3D for fucks sake.
The suits at Commodore literally dug their own grave, because they never fully understood what the Amiga was capable of, and as a result, they've completely ignored or neglected to look at their competition in the long run, as a result of their shitty business practices. There's really nothing else to it.