What I think is that - it never happens. The absence of the internet seems to be the worse thing happening in crypto as we rely on it.
In fact, it was been created because we are in the internet era which means that everything we do when involved in crypto trade comes with the use of the internet if not like you have said "Bluetooth" transactions, I don't think it really works. Does anyone work on this? Well, nothing because it was impossible IMO.

Some people say that the network signal is not good when you are away from home. If there are currencies that are traded offline, that would be great, and maybe there will be large-scale application scenarios for digital currencies! What do you think?