I generally use the collapsing templates for just about any large blocks of content that aren't necessary in order to understand what I've said; for example, I would collapse a block of code, a long list or a large table.
More generally, this template – and other templates enumerated in w:en:Template:Warchivenav (particularly the second-last section) – may be used, e.g. on w:en:WP:ANI, to collapse/archive completed discussions or parts of discussions (or, occasionally, rambling/irrelevant comments that don't add anything to the discussion), with this being done or undone by whoever thinks/knows it would be appropriate for them to do so. (The linked navbox template also contains a lot of other templates that are used on English Wikipedia talk pages.)
Could you clarify whether the first question is being asked in order to inform the development of additional built-in functionality (i.e. in order to supplant templates), or for different reasons? If the feedback is going to be directly used in the creation of new features, I imagine you would potentially get significantly different answers if you were to directly ask what new built-in in-text features would be useful; some things might not be possible with the current functionality of templates (or it might require a significant amount of additional investment to make them work with the currently available wikitext and Lua functions).
For me, the main difference between the Discourse and Reddit layouts is that Discourse employs a linear approach, whereas Reddit (and Wikipedia) use indentation to indicate replies. I think sticking with the latter approach would be the least disruptive (I imagine a Discourse-type layout would cause certain difficulties for processes like RFCs and deletion discussions), although there are definitely Discourse features that would be useful in a Wikipedia context (such as comment permalinks and the overview sidebar/scrollbar).
In the cases where I've notified someone while hiding their username, it's usually been to notify them that the conversation is occurring (for example, because their input might be useful) while avoiding additional emphasis on their notification. However, I've generally done this quite rarely, and the users who are notified in this manner often don't respond.
A more common use case for hiding usernames is exemplified by Wikidata's {{Ping project}}, which notifies users linked in the list of participants of a given WikiProject; the whole list is hidden and only the name of the WikiProject is shown. I think a more useful way of doing this, especially within a larger community where many WikiProjects would have more than 50 members, would be to formalize/build in that sort of functionality by having separate mailing list-style notification "inboxes" for such WikiProjects and other groups of users (similar to the group notification functionality in Discord servers); aside from avoiding the 50-user limit, this would perhaps allow moderation of the notifications to avoid spam, and allow a viewable public log of said notifications (e.g. on a new special page). (It's possible that this would be seen as redundant to MassMessage, but the redundancy would be in almost the exact same way that Echo is technically redundant to talk pages for the purpose of notifying users.)