Define big. They're one of the biggest companies in history. You're saying that if they appealed to 1% of the market that already uses Macs... what? And then they have to support these users?
Mind you that this (me dabbling around with it) was 2004, with most other GUIs still in stone age, and I think it was developed back in the Nextstep days.
As were Hypercard, Smalltalk and Lisp environments.
Interface Builder was originally a Lisp application, before being acquired by NeXT.
Note that BeOS wasn't POSIX compatible, that came with Haiku rewrite.
However without UNIX/POSIX to sell Apple to those frustrated Linux users looking for shinny UNIX laptops, and the management skills from Steve Jobs (vs Gassé), probably Apple would be no more instead.
Haiku has archived what BeOS programming looked like,
https://www.haiku-os.org/legacy-docs/programming_the_be_oper...