but we would have more than one OS as "standard". Write a program for OS/2, and then DR-DOS wouldn't run it. Write a program for Linux, and Plan9 wouldn't run it. Write a MacOS program, and BeOS wouldn't run it. Etc.
Best thing to do is crack open that Microsoft API and have other companies clone their own Windows to use that API, or build emulators or whatever to run Windows software. Then everyone can have a piece of the pie.
Remember in the early 80's when Compute! and other magazines had source code for programs, and each program had like 6 ports or more? One for each major machine?