I just had a look. I can absolutely understand parent. I’d want an option to include read messages in the inbox, not avoid marking as read. I want a history of stuff in my inbox, the same way discord, my RSS reader, and my email client work. All those have a read and unread state, but I can still see the read ones.
(And administrators can set it to be the default for new users in their organization, if the way your organization communicates is such that it's a better default than inbox).
