Well, I disagree with that. The HRC has a very nice magic collection, for instance, that has not "disappeared", and it actually very easy to access. Everything I've read so far about this acquisition leads me to believe that this collection will also be widely available. For example, see this quote from the president of the University of Texas:
"David Mamet's materials will be a great resource for students and scholars here at the university," said University of Texas at Austin President William Powers Jr. "He is one of the foremost and influential American writers and directors. I am personally pleased that his work is coming to the Ransom Center, because I have regularly had my freshman seminar students study 'The Spanish Prisoner.' "
In fact, over the next four years, Mamet himself serve a number of short residencies. As stated in the press release, "During these residencies, Mamet may be a guest lecturer in various courses, work with students on the production of a play and give readings, lectures and public addresses."
I think this is far from being hidden in some dark basement.