But see, that's the beauty of it. While there may be a shortage of 128GB modules, it doesn't actually matter -- because my hypothetical forward looking quote is almost guaranteed to ring true with most consumers. (It might even be true.)
And Apple doesn't necessarily have to increase the price beyond what the upgrade would currently cost in order to still win big. Just a few minutes of Googling suggests that the cost to manufacturers to swap over from 128GB to 256GB is somewhere on the order of less than $20 per unit, even in very small batches. (And that's an extremely liberal estimate, because Apple almost certainly pays way less than... well, everyone else.) Contrast that with Apple's current markup to make that same capacity leap, which is $100.