They use the vault the way diamond miners do- to restrict supply of extremely coveted goods so that they can charge premium prices for them. It's good business and it keeps things that are decades old in constant demand so they can charge higher prices for them.
Unfortunately for you and me that means that if we want access we have to pay more. While it may certainly be true that you don't place enough value on Disney products to pay a premium, there are lots of people who will and there are probably still going to be times that you want access to it and consider signing up. That's what they want- if they commoditized their content and you were able to get access to everything they own for $30 a month, that wouldn't serve their purposes at all. Companies like Netflix dream of having one franchise that's as successful as the countless ones that Disney has. Why on Earth would they just have a fire sale for all of their content when they can restrict supply and extract the most profit from it?