The meaning of the word masterpiece has shifted over time. As you say it used to mean the work that proved to your peers that you were ready to enter the profession as an equal. So certainly competent, maybe even considerably above average since so it marked an important event in the artisan's life, but still an early work.
Now it is more synonymous with magnum opus, among the very best works of an entire career. Perhaps ironically, with respect specifically to the dissertation it is a common mistake to try to create a masterpiece in the modern sense rather than the classic sense. Probably because every field has legendary careers that were launched with the publication of a game changing dissertation. This is one thing that can cause ABD to drag on forever.
Now it is more synonymous with magnum opus, among the very best works of an entire career. Perhaps ironically, with respect specifically to the dissertation it is a common mistake to try to create a masterpiece in the modern sense rather than the classic sense. Probably because every field has legendary careers that were launched with the publication of a game changing dissertation. This is one thing that can cause ABD to drag on forever.