today 50TB is expensive

I don't believe you can go back more than five years and find per-TB rates under the current rate. Ten years is straight out. Twenty years is laughable.

Meanwhile, PBS has an annual operating budget of around $340M. They can afford to manage their own data, at the scale of tens of terrabytes. And I guarantee you that whatever they're paying Iron Mountain to host the data is multiples the cost of self-hosting.