Israel and Iran both shut down their internet during the ongoing conflict for multiple days. It's standard operating procedure to flip the "internet kill switch" as soon as a foreign bomb explodes on your soil. In a major city like Tehran, Tel Aviv, NYC, SF, London etc where you have 10,000+ people per square mile, you could easily have 20-30,000 people on a network like this with moderately low latency during a bombing situation.
But yeah if you live in the Houston suburbs where each house sits on a half acre of land and houses are 150' apart, ti would be useless, you're right. Very few people outside of the US actually live like that, though. So yeah it's useful for about 55% of the global population.
I dunno how you cache and forward messages for more than maybe a thousand people or so, encrypted data by definition doesn't compress hardly at all, that app might use a gb or more of storage if you're at the edge of two networks.