Huh, according to @logankilpatrick in Slack and the value of community heritage - #23 by logankilpatrick, the messages don’t go into the void. One can still download the whole archive. The problem is that old messages don’t show up if you search in Slack’s search bar. It should be possible to host this Slack archive as a static server, but then the content becomes web public, and is prone to search engine indexing.
The main point of the Zulip mirror is to make the Slack messages searchable and at the same time not web-public. The mirror is unidirectional.