It’s concerning to me that 90%+ of these videos are hosted by a single entity. The significance to me (and I assume many others here) is a cultural one. These videos relflect on our livelihoods and our day-to-day interests and pursuits.
My advice is to not get complacent about always having access to this content. Use youtube-dl and keep a local backup of what’s important to you.
Concerning, maybe, but the same could be said of a lot of other centralized mediums (such as Medium, WordPress, even HN threads itself).
When you rely on someone else to host your stuff, you always run the risk of it disappearing. Important stuff should always be backed up, whether it's video stored on YouTube or an amazing blog post on medium.
If YouTube disappeared tomorrow and all these videos disappeared, I'm sure within a week or month, people will have reuploaded many of the really great talks posted in this thread on some other site.