There's no particular reason the single standard platform (facilitated by IBM being incompetent both in publishing specs and writing their contract with Microsoft) has to win.
I don't think it matters hugely before the advent of GUIs. One DOS or other text interface is as good as as the next. Businesses will probably pick a single platform (IBM/MS-DOS I imagine) and consumers won't care terribly.
The situation will be much like before the rise of IBM clones: Apple II, Commodore 64, IBM PC, and the Atari 8-bits (it's possible that the British Sinclair ZX Spectrum becomes more popular overseas as well) all using proprietary hardware and software.
To that end C-64 will likely dominate and continue to dominate the low end, IBM the business arena, and Atari and Apple fighting it over the rest.
It's possible that Compaq, Dell, and others develop or buy their own OS and set up in business for themselves as well.
What changes the market is the arrival of Graphical User Interfaces—Lisa, Mac OS, Windows, and probably more.
What happens? My best guess is a rapid proliferation of GUI operating systems still tied to single manufactors, and an attempt by others to push down, and finally a NeXT analogue attempting to make a multi-OS/CPU application framework (aka *OpenStep) which makes for somewhat easier/more common multiplatform programs.
I'd expect Microsoft Word (later Office) to be hugely important in the ATL, as it will work at least for Windows and Mac OS and probably also for Commodore Amiga (assuming in the ATL Commodore does indeed wind with Amiga for a GUI) and whatever else is out there.
One might expect to see more companies enter the business with custom operating systems. I could Sony (or another Japanese company) buying NeXT or BeOS for example or developing their own alternative.
I could also see some companies—Sun?—move their server OS downscale into the consumer market in the early '90s.
Overall post-GUI computer sales are probably a little slower, and you might see more quasi-computer alternatives like consoles/WebTV/handhelds do better earlier.