This dramatically undersells what MacOS is and was. It was way beyond just a window manager.
From its inception in the 1980s it included a set of APIs that allowed developers to build sophisticated (and consistent) GUI applications with comparatively little effort. eg Quark Xpress, Illustrator, Photoshop, Excel, Word
By the end of the classic Mac era in the late 1990s that API set had grown to include a ton of stuff. QuickTime, ColorSync, TrueType, AppleShare, sophisticated printer support, multiple display support, etc
The facts are: The only other contender was BeOS, after Talligent flopped and Copland imploded.
But Louis-Gassée overplayed his hand.
Source: all of the (other) Steve Jobs books
Apple had less resources, especially in the dark 1990s, to support such a move. It was made even worse by the fact that its leadership was probably not even aware of the difficulty in moving over, as well as the fact that 1990s Apple wasn't exactly a place people expected to "change the world?".
Hence becoming Jean-Louis Passé.
Going to use alternatives like Haiku that can access many modern systems but on such low powered hardware shows what wastage we have.