If someone pays $750 for a 64GB phone, they can't complain that it costs them $750GB for the same 64GB next year, regardless if the base model doubles. You buy 64GB because you need ~64GB of storage. If you can get by with 32GB then many people can actually get a $100 cheaper phone the next time.
Of course they can complain. Its all about incremental value for the additional $100 you pay.
What if the base model was 63GB.
Do you think 64GB users would be happy to spend $100 for 1 additional GB of storage?
That's an extreme example but it does illustrate the real math people use when they decide to upgrade or not.
People compare the prices and what they get for the additional $100 or $200. And they will see they will be getting less incremental value than last year. It makes no sense to punish your most valuable customers who buy the high tier phones.