Atari ST was crushing Amiga for years until Commodore got their stuff together and bifurcated the product line into the Amiga 2000 and Amiga 500 while also getting good games on the system - plus the Video Toaster and other accessories to finally take advantage of the multimedia superiority of the platform.

It didn't really matter in either case since IBM and its clones were destroying both of them with Macintosh stealing the non-IBM market share all the same.

The ST didn't use CP/M-68K (it did during early development, but pivoted to a newly written file system and loader before it shipped).

The ST failed because the Tramiels didn't really understand software platforms. They got hardware, but when it came to developing an ecosystem that could support developers over a long period of time they just couldn't compete with the likes of Apple and Microsoft (the ST's software team was 10-12 people, and I don't care how good they were, doing a long-term successful platform just wasn't going to happen with a team that small). Coupled with some predatory business practices, the writing was on the wall.

The Magic Sac was more of a curiosity than it was a serious solution to anything. I have a great deal of respect for David Small, but the product was never going to work for the 68020 and up Macs.