Good discussion. Sorry about my bluntness, but:
User list will be in core, just like the discussion list. It will have plenty of extension points so extensions can add/change information and functionality. It will most likely be a part of the main forum interface (rather than admin CP), with extra functions integrated for admins, as @Dominion has alluded to.
Whether or not a feature will be a part of core vs. a bundled extension is a philosophical discussion, one which is at the core of Flarum's direction and that I've obviously thought about a lot. But this thread is probably not the place to have it. Still, I will quickly and specifically reply to Kulga's points...
I think you are making a couple of incorrect assumptions:
That the core user list will need to be replaced by a completely new user list in order to be useful. While it no doubt will be quite basic at first, it will get more powerful over time, and it will contain extension points so extensions can add their own power.
That if you do want to replace it with something completely different, you'd have two user lists running side-by-side. In fact, you could just overwrite the default user list. Flarum is a JS app, so you can just associate a new component with the route. Only disadvantage is that the JS file would contain a dozen or so KB of unused code.
Whereas, the disadvantage to having a bundled "basic user list" extension is that you end up with dependency hell (again as @Dominion points out). In my eyes there is no philosophical difference in wanting to extract the discussion list into a "basic discussion list" extension.
Please create a new thread if you want to continue this discussion đŸ™‚