> existential threat from rights holders due to users uploading content that they do not hold the rights to

Said threat only comes from big content owners, and they don't even bother to check if they have the rights to the content before issuing a DMCA challenge. A small-fry content owner doesn't have the resources to find copyright infringement, while big content providers simply scrape everything with automation and issue a claim on even the flimsiest match.

At YouTube, big content doesn't even have to do the work, they've pushed the work off to YouTube's content ID system which automatically demonetizes (or, better yet, gives the revenue to the claimant).