"Maybe I didn't see where the Mariposa could be bought in a more robust material."

My bad ... I looked just now and I don't see a straight Mariposa sold by GG anymore, just the "plus". I'm sure that both models were sold last year; I'm guessing that folks that went for the Mariposa model mostly went for the "plus", and others went for one of the other models (Miniposa, G4).

Maybe this, but definitely the advent of the GG Gorilla model seems to confirm --- at least on the GG site --- your feeling that packs are going back up in weight. For the smaller companies like GG, I would guess that this is very much customer (demand) driven.
I'm sure the Gorilla fits some particular niche, but for a light pack with stays, I still like the Mariposa Plus better. I have no direct experience with any of the current alternatives.