phone users won't become relays. that will just kill the battery. phone users running their own relays at home in some way would work though, the same way we run lightning nodes on the pc, not at the phone. if not i2p, maybe, we could use torrents&magnet in some form? free message sizes could be limited to encourage this
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You'd be surprised with the efficiency of push notifications.
I've got a personal nostr relay running on my phone, uses like 3% of the battery across the day.
This happens because there is no display involved, which tends to eat the most energy.