[mscrnt]

Not a replacement inasmuch as it's not a standalone chat client to be used by disparate users for various reasons. It's a chat client for developers of a given project to convene discussion about work going on in that repository at that point in time. So where Discord, for example, is for anyone to setup a server to discuss anything; Fossil chat is for devs working on projects that already use Fossil to convene discussions pertaining to that repository. You won't setup a Fossil instance just to use as a chat client—because it's not intended as a replacement or competitor for <list of chat platforms>—but if your project already uses Fossil, now you can keep your dev-related chat in-house.