well unix existed in flavours available to the personal computer market. since 85-86 a personal computer was just as powerful as a minicomputer from the 70's. UNIX was multiuser and based on a highly successful language.
The first flaw Amiga suffered for me is that was another set of formats (filesystems, os, libraries, hardware) in a world that had in 1986-87 (when Commodore started to push it seriously) already highly solid standards (and many in the software world.). You can then and today how is difficult to port an application from the open source world to Amiga. It was a nice OS but too much linked to the hardware (this was an old background scheme). I can't remember of any app in the Amiga World that also the other users (pc users, mac users, unix users) widely know. Yes there were good apps but no one of them became a standard that's for sure...(like Lotus, Photoshop, Word, Excel, Borland DB, emacs...)
can't take away the fact AmigaOS was a nice original OS...