Worth mentioning that "allow" vs "block" aren't the only two options. There are "limited" settings, where users from an instance can follow & be followed but don't show up in more public content.
https://docs.joinmastodon.org/admin/moderation/
So it is possible to imagine a set-up with an allow-list of known well-moderated sites, a block-list of known bad actors, & limited setting for new/unknown instances (maybe with new limited settings that work better for cautious discovery of new servers).