TwoSix
Everyone's literal second-favorite poster
I'd say that fluff and crunch exist on a bit of a spectrum. Like, if I said that all humanoids in my campaign world had 6 fingers on each hand, that's pure fluff. If I said that magic requires intricate finger motions such that only humanoids can do magic, then I've made fluff-crunch. It's a story element that can have mechanical repercussions, like cutting a finger off of a magic character. If I then said that "Only humans have 6 fingers", then I've made a crunch change (only humans can be magic-users, not any other race) with some fluff justification.That's why I said it's not JUST fluff. Most of it is fluff(how you describe it), but no "weave" = no magic. No magic = crunch. An anti-magic field isn't the "weave" shutting down, it's a different mechanical way to represent the loss of magic.
Edit: Mystra getting a headache = fluff. Turning off the weave = crunch. All magic goes with it.
I tend to view the Weave as pure fluff as there's simply no way for the players to do anything with it without a bunch of DM intervention. If the DM gives you an artifact that lets you manipulate the Weave somehow, than it becomes fluff-crunch.