You state that Commodore later released machines with expansion slots. That is correct, but Atari also released machines around the same time with slots of their own — the MEGA ST series. One of the most popular expansion boards for the MEGA was the Crazy Dots video card.

I also am sure the Amiga played much of a role in the demise of the ST; Commodore went under and the Amiga disappeared in 1994 — Atari was still selling Falcon030 computers that were considerably better than the comparable priced Amiga 1200s that Commodore was flogging up to liquidation.

Both Atari and Commodore had difficulty keeping pace with the updates to the Apple and (especially) Wintel worlds.

Atari actually did a better job than Commodore in many fronts. There were two ST portables/laptops; Commodore never managed to make an Amiga laptop. The VIDEL video chipset of the Falcon was a quantum leap ahead of both the STs and Amigas of previous eras and ran circles around the Amiga as well as consumer-priced 68K Macs of the era.

The problem with the ST (and Amiga and almost Mac) came down to people wanting a single platform to develop for and use. Tech loathes diverse platform ecosystems; just as the later smartphone market would go from a dozen players to just Android and iOS, the PC market shook out the ST, Amiga, Acorn, and other platforms to land on Windows and Mac as the dominant players.