This estimate is also an overestimate according to the paper.

First, much creative endeavour builds upon the past and an extension of term may make it more difficult or costly do so. Were Shakespeare’s work still in copyright today it is likely that this would substantially restrict the widespread and beneficial adaptation and reuse that currently occurs. However we make no effort to incorporate this into our analysis despite its undoubted importance (it is simply too intractable from a theoretical and empirical perspective to be usefully addressed at present).

This means that the real number is significantly less than 15, maybe more like 12.