Why would anyone that has that amount of mail go out of their way to get a phone with such limited capacity. At this point a phone with 16gb serves the purpose of being a burner phone.
There is no limit on gmail. For someone who has it for a decade or more, 20GB mail is realistic, since gmail always told you there is no need to delete anything, ever.
As for 16GB phones, those are pretty normal; take a look at moto e5. Please don't tell me a 64GB phone is required for email.
It also doesn't change the initial problem of what and how much should be cached by service workers.
