How long did Apple keep selling 16GB as base storage for their flagships? That should be a sign. Part of Apple's pricing is not just on BOM cost, but it's on the brand (disregarding any recent issues). Many people buy iPhones because of the brand, just like how people buy Nike shoes, LV bags, etc.
And yes, it would really be annoying if Apple is keeping 64GB as base storage. That's fine for the SE, but for the main flagship, 128GB should be the standard now. I bet knowing Apple, they will keep 64GB as base, but they will bump the next tier to 256GB. This will nudge most people to just get the 256GB model, meaning higher ASP for Apple. I believe Apple did this before with the 6s, where the base was paltry 16GB, and the next upgrade tier was a big jump to 64GB to nudge people into spending more.