The most important difference a Usenet-like clients brings is per-message read/unread status, which is what enables long-running threads and discussions. Without most clients (most users) supporting such read/unread tracking, you won’t have long-running discussions, which changes the whole character of the medium.

Per-message read/unread status in practice requires keyboard navigation (or as an inferior alternative, paging with read/unread tracking like in web forums), which doesn’t work for mobile.

The lack of read/unread tracking is also why we don’t have long-running discussions on HN.

The only remaining medium with per-message read/unread tracking we currently have is mailing lists.