Why Berne Convention has chosen a 50 year term for copyright

Reading the very sensible Guide to Berne Convention by Masouyé, WIPO's Official book on the convention:

7.4. It is not merely by chance that fìfty years was chosen. Most countries have felt it fair and right that the average lifetime of an author and his direct descendants should be covered, i.e., three generations. Clearly the justice of the period varies; it depends always on the length of the author's life and the difference between cases in which he is cut off in his youth or becomes a centenarian cannot be avoided. But it is generally felt normal to add to the author's lifetime a period long enough to allow his heirs to profit from his work while they remember him. Experience has shown that, when an author is dead, his works sometimes fall into a sort of limbo from which they may or may not emerge some time later. In any case, apart, perhaps, from books and certain dramatico-musical works. modern means of exploiting works often make the length of the term of copyright of little financial importance to the users: the latter negotiate blanket licences with the authors' representatives to use large repertoires and normally the lapse into the public domain of any given work makes little difference to the amount they pay. For all these reasons, this minimum period laid down in the Convention seems to provide a fair balance between the interests of authors and the need for society to have free access to the cultural heritage which lasts far longer than those who contributed to it.