I fully intend to continue this series. But before proceeding, I thought it important to establish the correct girsa of the Rambam. In an earlier post, I noted that Mechon Mamre's Mishna Torah, which is based on the majority of Yemenite manuscripts, had no leading vav in בשלשה עשר. And this had major relevance in terms of determining peshat in the Rambam. Namely, that he is not saying that all four fasts are minhag, as well as Taanis Esther, as Maggid Mishnah and Beis Yosef take it. Rather, only Taanis Esther is the topic.
Since then, I looked it up in the Frankel Rambam, and he notes that without the vav is what exists in the dfus rishonim and in the manuscripts. And from JNUL, I looked up such a manuscript. The relevant text begins six lines from the bottom of the image to the right, and indeed there is no leading vav. See it in context here, page 87. This text was from 1480.
Based on this, I believe that one would have to conclude that Maggid Mishnah is wrong. One is free to disagree with me on this, but from where I stand, it is fairly clear.
And so the Rambam never explicitly says that the reason we all fast nowadays is minhag. And this leaves it open -- it may indeed be because of minhag, but it may well be because the period of reshut only existed when the mikdash was standing, a position which seems clear from his peirush hamishnayot.
Bli neder, soon to come is a discussion of Rosh, how this may be a viable alternative to Ramban, and how this perhaps influences Tur.