Palm couldn't build the iPhone with Palm and had to build WebOS. QNX is amazing, but its UI and client developer library wasn't there or else RIM / Blackberry would have used that. Apple had used Linux, and frankly, there was no way Apple would have done a Linux-based system after the whole GNU thing at NeXT.
I do think Objective-C, the developer tools, and NS class libraries don't quite get the credit they should. They were very good (go read the Taligent documentation). Truthfully, if Adobe and Microsoft hadn't insisted on Carbon[2], the Mac would have been much better off.
Apple had done an Intel translation internally and had the institutional knowledge already from the 68K to PowerPC transition.
1) https://www.youtube.com/watch?v=a3MrpLyUOo8 - its pretty amazing some of the statements, guess the clone thing changed
2) not absent the Classic layer, just absent Carbon.