Does YouTube have any incentive to “preserve” anything once it costs them more money than it brings in? If not, maybe it's a little early to say they are actually preserving anything. (The article does kind of acknowledge this.) Maybe its real role is as a link in a chain that leads to /r/datahoarders
It's totally worth them storing the data given the marginal cost.
A 10TiB HDD is only 150$ retail. These videos can likely be stored in less than 200MiB on average.
Storage has gotten cheaper YoY