Agreed. Once mobile data coverage is universal (via starlink et al, maybe), it's inevitable that the idea of a phone number will become antiquated. Either whatsapp (or one of its competitors) gets a sufficient monopoly and enables easily portable identities (to allow switching sims), or some other similar platform will come along. It may take a decade or two, but it will happen.

But it's never going to be universal. I felt very scared some weeks ago during a huge march against rightwing extremists in Munich, Germany. There were ~150k people concentrated on a few streets/km.

Now, how is this relevant? Well, the entire cell network was offline, at least for some providers. At first it wasn't possible to send/receive data. Calls were connecting, but my friend sounded like an alien. Then for one hour, 0 communication was possible.

So even though the most efficient (I think?) protocol was used, it came to a halt

I used to think this about email also.