There will never be universal coverage. Even in the largest cities there are dead zones.

A car rental business that assumes 100% Internet access availability discovered the problem of customers parking the cars in parking garages (signal dead zones). They did not have an offline activation mode and had to tow the car. When any given customer triggered too many tows, they fired the customer.

The PSTN is not going anywhere. You can choose to cut yourself off from it by refusing to take calls unless it's via an app.