Being earlier to market seems like it would have made a huge difference in competing with the IBM PC. I don't know if the dominance of the PC was a foregone conclusion by 1985. If the Amiga and ST could have been relased in 1984 it might have made a difference. Either way it's fun to think about counterfactuals.

I think the Amiga could have been released at least 6 months earlier if not for the litigation between Tramiel's Atari and Commodore gumming-up the works. The Amiga was massively more capable than the Macintosh of the day-- with preemptive multitasking, color, and NTSC video compatibility. There wasn't a Mac that held a candle to to the OCS Amiga graphics until the Macintosh II in 1987. I think a stronger Amiga marketing presence could have killed Macintosh.

VGA didn't come to the PC until 1987. The Amiga easily exceeded the capabilities of EGA. Windows 1.0 was released in 1985 and is much less capable and sophisticated than Workbench. I don't know much about the evolution of the native Amiga productivity software market. Failing the growth of that market in this counterfactual world arguably the A2000 and a bridge board was probably the first Amiga that could have seriously taken-on the IBM PC. That wasn't until 1987 in our timeline. It might have happened sooner in my hypothetical, but probably not years sooner.

Atari could have marketed the Amiga better than Commodore ever did. I recognize that's not really saying much. The post-Tramiel Commodore was an adrift ship of fools when it came to choosing which products to produce and how to market them. Commodore couldn't figure out what to sell (and how not to cannibalize or compete with their own products) or how to sell what it did produce.

I'm less sure what would have happened if the ST had been released as the Commodore product it arguably was. I do think a world where Tramiel stayed w/ Commodore would have made them more focused. They would have have developed fewer products (and likely no products that competed with other Commodore products) and aggressively driven costs down. I think a Commodore with Tramiel still heading it up would have been in a better position, financially and in terms of market perception, to put out a GUI-based business computer than Atari. I'm just not sure how much sooner I'd want to speculate a Commodore ST could have been released in this hypothetical.

The ST was also positioned more as a Mac killer than a PC killer, like my hypothetical Amiga.

You're likely right, though-- by 1985 the IBM PC was probably destined for dominance.