Does this mean that I need to wait until September to reply? /jk
I believe that the problem with the neolibs in this case is not the descriptive model (tragedy of the commons) that they're using to predict a potential issue; it's instead the "magical" solution that they prescribe for that potential issue, that "happens" to align with their economical ideology, while avoiding to address that:
in plenty cases privatisation worsens the erosion of the common resource, due to the introduction of competition;
the model applies specially well to businesses, that behave more like the mythical "rational agent" than individuals do;
what you need to solve the issue is simply "agreement". Going from "agreement" to "privatise it!!!1one" is an insane jump of logic from their part.
And while all models break if you look too hard at them, I don't think that it does in this case - it explains well why individuals are buying DRM-stained e-books, even if this ultimately hurts them as a collective, by reducing the availability of DRM-free books.
(And it isn't like you can privatise it, as the neolibs would eagerly propose; it is a private market already.)
I'm reading the book that you recommended (thanks for the rec, by the way!). Under a quick glance, it seems to propose self-organisation as a way to solve issues concerning common pool resources; it might work in plenty cases but certainly not here, as there's no way to self-organise people who buy e-books.
And frankly, I don't know a solution either. Perhaps piracy might play an important and positive role? It increases the desirability of DRM-free books (you can't share the DRM-stained ones), and puts a check on the amount of obnoxiousness and rug-pulling that corporations can submit you to.