Lets ignore the fact that Mac, Linux and Windows were developed over many years with hundreds or even thousands of developers working full time while Amiga Inc. have had a couple developers (full time? part time?) working for the past two years. Forget about that. Imagine (if you can) that OS 5.0 is coming out today - right this second, completely finished, totally awesome looking and running flawlessly.
What software does it run? Does it run existing Linux, Mac or Windows software? (I assume not) Does it run existing classic Amiga software? (probably not) Does it even run existing AmigaOS 4.0 software?
Lets (generously) imagine it runs existing OS 4.0 software perfectly with no conflicts or crashes. So what does it run - a few freeware games, a text editor, a few miscellaneous home-coded utilities. Is there any "professional software"? (or even anything as good as the out of date software on classic Amiga?) Is there any good software company planning on writing software for OS 5.0? If so, and considering how long it takes to write "professional software" hadn't they better start now? Are there APIs and tools for these software companies to actually develop software for it?
So consider:
Even IF OS 5.0 was out TODAY and ran flawlessly on modern hardware with modern graphics cards and modern CPUs, and even IF Bill McEwen had somehow convinced major software companies (tempted with "huge" potential Amiga market;-)) to sign up and develop for OS 5.0 and there are existing tools RIGHT NOW for these major software companies to start developing real and useful software - how long would it take? 2 or 3 years? So lets say that for 2 years, you have an operating system with only small utilities and freeware games. Who in the hell would be interested in buying this thing? How would a market be built up? What major software companies would be stupid enough to spend years of development time on an unproven operating system with no market?
"Oh, I'm sooo excited about OS 5.0."