First, I am not a fan off integrated training for many reasons (some of which you pointed out).
Second, I have mixed feelings about removing ANY military traditions, especially in the Marines and Navy (aside from my own service, I have skin in the game: my father, my grandmother, and several cousins have been in the Navy and Marines...and in this purpose, I do not hold distinction between the two).
That said, tradition is peer pressure from dead people, and while it holds a significant place, that simply cannot be an argument for why an organization or institution chooses to do something. In fact, arguing tradition is part of the downfall for many organizations, institutions, and cultures. So there must be a balance between tradition and evolution.
Having served and grown up on many fine Marine bases in base housing not fit for an animal (here's looking at you, Lejeune!), I err on the side of history and tradition, and based on my personal beliefs, I think it unwise to git rid of either PI or MCRD SD much less integrate in boot.
Regarding history, your first point is historically spot on (re: 'old breed'), but there remains historical ambiguity about 'devil dogs'; there are some sources that it was created by the mighty USMC PR machine (which, truthfully, is without peer....).
But here's a question: what makes a Marine, a Marine? Asking you to get philosophical. Core values? Esprit de corps? Narrow and rigid culture?
