I also opt for P2P whenever I can, but it does have drawbacks as well, besides NAT. First, both have to be online at the same time. Second, seems many connections are still not symmetric, so sending files are limited by the upload speed of the sender, and together with the first problem, makes the downloaders experience suffer because of the uploader.

With that said, better infrastructure (IPv6 + better connections) can make P2P very feasible in the future, hopefully. Or software that defaults to local connections if it's possible (so if we can find the device via a private IP, use that connection instead).

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.

Is P2P awful? I've been using Resilio Sync (neé Bittorrent Sync) for a decade now; if there is a reliable piece of software in my stack, this is it! Sharing files is a matter of passing someone a code. It works so well I really don't think about this as a problem.