I think having “Active” as default is unfortunate too, if a new user sees the top post was from two days ago they might assume the whole platform is inactive, since they can’t see that a new comment was added just seconds ago from that page view. Having “Hot” as the default would probably be better since those are usually newer posts with at least a few comments.
Maybe instance owners could have a recommended subscriptions option that new users could default to. That way new users can experience seeing content on other instances right away. Different instances with different standards could customize their new user experience.