Not much, really. I would even dispute the premise: the Commodore Amiga was still a viable platform up through the mid-'90s, for instance, and Atari's ST was still around and kicking then too.
Much or most of the Amiga and Atari ST installed base in the '90s was "low quality" though: users who didn't have hard disks and likely couldn't afford one, who were possibly using a TV and likely couldn't afford a decent monitor, who didn't have a printer and who were mostly buying games that came on booter floppies https://en.wikipedia.org/wiki/PC_booter . Those guys would have had trouble using business/"productivity" software effectively even if they'd been interested in buying it. Obviously there were better-equipped Amiga and ST systems too, but by and large Apple and the PC had run away with the users who had the budget and the inclination to run application software.
(In general, the serious cost barrier of a hard disk and a decent monitor as more or less the price of entry to serious computing almost certainly had a major impact on the shape of the computing industry until pretty recently.)
If a few -- and only a few -- things had gone differently, Commodore could be Apple. The Amiga was ahead of its time and was in many ways far better than the Macintosh (at the time, or even a bit later).
Apple succeeded because it found two niches to sustain it -- education, design. Both Commodore and Apple had small but enthusiastic base of fans, but it's been shown time and again that fans alone are not enough to sustain a whole platform or company.
Amiga was well represented until the late 90s when the print magazines died off and that was the death knell. Both Amiga and Atari were in Computer Shopper too, albeit token pages.
