This already happens. Sites that vest their entire survival in user-generated content take a big risk, because the brand will become associated in people's mind

This already happens. Sites that vest their entire survival in user-generated content take a big risk, because the brand will become associated in people's minds (though personal experience or hearsay) with the behavior of particular elements in the community. While people will have different experiences, some good, some bad, the reputation of the site will keep some away. Platforms that host communities that are editorially distinct -- like Discord, Reddit, even 4chan -- insulate some of the mayhem that radiates out of one channel from completely spoiling others. Tumblr (and Twitter) have no such mechanism, they don't have "Channels" or "Groups", all content is propagated through follows, reblogs, and un-namespaced public tags. Communities exist solely in the minds of their participants, but are not enforced or enabled by any platform feature, and thus are both trivially infiltrated, and offer no protection from the goings-on of the wider ecosystem. The only way to seek refuge from this to leave the platform altogether.