Considering its price, the Atari 520ST was a good machine. The most common PC of that era ran an 80286 CPU with MS-DOS. However, in the end, the Atari, Amiga, and Macintosh could not keep up with PC clones, which were innovating much faster. Apple just got lucky and survived to 1997 because of its loyal DTP user base. Then, Microsoft saved it because it didn't want to be perceived as a monopoly.

At the time the Mac launched and for many years after Apple continued to bring in big coin from its Apple II series sales (which was very successful in schools), not the Macintosh. So it wasn't really DTP that was keeping it surviving, at least for the first few years. Obviously that changed by the late 80s.