Why not just have uhttpd on every phone, and I can share a file to it, which adds a symlink to the file in /www and gives a QR code that I can scan on any device on the same network to download it over wifi?
I don't think it quite works how airdrop does (I don't use the Apple ecosystem so what do I know?) but it would at the very least allow relatively seamless, secure, peer-to-peer file sharing between phones on the same Wi-Fi. Might be a lot for users to figure out but still not as difficult as setting up mobile pay, and then once they've figured it out, they've got it forever.
