I don’t think that’s the causality. Still-existing web forums, mailing lists, places like HN and (to a lesser degree) Reddit show that it is possible to have public forums without them being overrun by spam, bots and trolls, given adequate moderation. This is particularly true for special-interest topics. (The dynamics tend to be a bit different in bike-shedding territory.)

The reason Usenet died is because it didn’t have an easy moderation system, and because web forums were more accessible (no special client software needed). Mailing lists died because people moved from desktop/TUI mail clients to web mail, which didn’t provide good usability for handling mailing lists. Web forums died because they didn’t translate well to mobile and because chat-like platforms appear more accessible to novices.

Mobile plays a major role here. Mobile is more accessible (approachable) than desktop software/hardware, and at the same time is ill-suited to long-form discussions because of the small display and worse typing experience. Platforms want to maximize their audience and therefore tailor their application/functional design to work well on mobile. As a result, their design is ill-suited to long-form discussions.

Public discords are no better for bad actors. You just need active moderation in both cases