I think your reasoning is exactly why people get pissed though. When technology advances, previous tech becomes cheaper, obviously.
That's why, like you said, 32GB was doubled to 64GB for the same price. 64GB was doubled to 128GB for the same price. So following that logic, why can't 16GB be doubled to 32GB for the same price? The answer is that it can, but Apple knows that the iPhone will sell just as well whether it does or not.
I personally think it's just plain silly that a flagship, $650 device has such a low amount of storage, and are the last flagship smartphone maker to still hang on to that low amount. True, not everyone needs a ton of storage, but why design "the most advanced smartphone in the world" around those who aren't going to use it's features that much?
I know I'm beating a dead horse here and again, it's about voting with our wallets, but it's just my 2 cents on the matter.