The difficulty created by NAT and IPv4 is drastically overstated. Even if users' devices generally had public IPs, users still wouldn't want to install server software nor leave their computer on. There is no money to be made pushing solutions that cut out the middlemen, so no advertising continually telling people "Try FooTransfer", and thus no network effects. Instead, one user goes "I can send you this using FooTransfer" and the second user goes "that sounds scary and hard".

And doesn't Dropbox work for the given example? That's the mass-market productized/paid/marketed/surveilled solution.