Instead of looking at it from the perspective of a power-user, look at it from Apple's perspective: Apple has hundreds of millions of customers, and they have the diagnostic data from all the ones who consent to sharing that info during the phone's setup. Apple knows what its average user is like in terms of screen-on time, app usage, storage, etc.
They haven't increased it since they went up to 16 GB in 2009 with the 3GS, and since then they've gone on to introduce several 8 GB models as "budget" models. Apple's business relies on providing good user experiences, and if most of its 8 GB and 16 GB users were maxing out their storage on a regular basis Apple would notice, their reputation among the average user would suffer, and they'd have to increase the base size. Since they haven't increased it in such a long time then it's safe to say that Apple's average user isn't maxing out their storage on a regular basis.
Remember, most people posting here aren't Apple's average user, we're power users. For power users they've doubled the storage while keeping the same price, so users who used to get 32 GB now get 64 GB, and users who used to get 64 GB now get 128 GB.
Basically, the people who historically haven't used much storage, "average users", *still* don't use much storage. Power users, on the other hand, the ones who used to be okay with 32 GB and eventually 64 GB, are now using even more. We'll likely only see the base model increase to 32 GB or 64 GB once Apple's "average user" (again, not power users) needs it. It could be next year, it could be in 2020, but only Apple can really predict that.