The Super Bowl ad mentioned 500k mi^2 of the US are not covered by cellular, which is about 13% of the area of the US. Looking at the Verizon, AT&T, and T-Mobile coverage maps it does look like somewhere in the ballpark of 13% is uncovered, with most of that being in Alaska.
The ad describes these places like this: "Places where emergency texts never send. Where emotional messages are not received. And countless memories are left unshared".
For the emergency texts, that is something clearly useful to anyone who finds themselves in a place without coverage, regardless of whether they live there are or just visiting.
For the "emotional messages" and "countless memories" what the ad showed as examples such as video of a high school basketball game dramatic win at buzzer, a baby's first steps, and a child's first bike ride without training wheels.
Those would be useful for people living in the cell-free areas, but I'm not sure they would be that useful for those just visiting. If I'm vacationing in a remote wilderness I probably only want emergency communications to/from outside that area.
I'd guess then is that the market for this would mostly be people living in those areas. Maybe people visiting will get it short term (if that is allowed) for the emergency texting if they aren't going to be staying with people who live there and already have it.
So...how many people live in such places?