Once there are universally reachable cellular data networks, the days of the phone network are numbered. A "phone number" is an antiquated concept, and the network has been ruined by spam. Messaging apps will prevail.
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.
Bold claims. You're willing to bet this is still the case in 20 years?
> A "phone number" is an antiquated concept
As much as I wish that was true, my observation is that phone numbers aren't going anywhere, both as an addressing and authentication layer.
> as an addressing and authentication layer
Phone numbers are a horrendous authentication layer. That'll hopefully die sooner than phone numbers for addressing.