We could have gone the slightly-less-compatible route with CP/M-86. Network effects would make the industry gravitate towards a standard platform, but not necessarily a company.
I remember tons of GUI software running on proprietary Unixes on vastly different architectures, all written aiming POSIX compatibility and an X graphics environment.
Yes, CP/M-86 could have played to role of MSDOS but people really needed to buy a box with a floppy disk to install from or a preinstalled OS on their new PC. MS did that right.
All that is not very relevant. Had Microsoft never existed, it'd have been an ecosystem too different to make any valid prediction. We'll have to eventually restore the universe and run a different path to see.
Just like it happens with any standard, we ended up with lots of #ifdef, because many APIs might had the same name, but not the same semantics.
And like in UNIX like OSes, all the cool goodies weren't in the POSIX APIs anyway.
It is quite sad that despite some of the cool GUIs like on NeXT and Sun OpenWindows, the best that they could settle on as a standard was Motif.
Well, we did have Richard M. Stallman, Emacs author, not only create an OS (GNU), but also extend Emacs to the point that it might as well be an OS. More pointedly, Miguel De Icaza created Gnumeric (a fantastic Excel clone) before he started the GNU Network Object Model Environment (GNOME).
Corel attempted something similar to what you were saying, as well; Their Wordperfect/Office/Draw empire was leveraged against Microsoft's Embrace/Extend/Extinguish in order to promote their (debian-based) linux distribution.
However network effects would have given to another company the position that MicroSoft enjoyed for some 30 years. It could have been a slower growth or a faster downfall, but somebody would have been there. Not Apple because they are interested only in selling their own hardware. Probably some company out of nowhere, not necessarily on a x86 chip. Then everybody would have started building drivers for the OS of that company and Linux and MacOS would be running on the preferred chip of that OS.
Maybe in an alternate reality Visicalc built their own OS to run their spreasheet on a large screen (by the time standards) 8086 computer and replace MSDOS and Microsoft as the OS and sw company of choice. We'd be talking about VisiOffice and Vindows now :-)