The "with port forwarding" is the problem there. That assumes you have access to your NAT resolver to add the rule, which isn't a fair assumption to hold. People at colleges, businesses, hosted events, etc. need to do NAT traversal (or be on IPv6) in order to do P2P.

I didn't see that you mentioned UPnP in the OP though, sorry. I'd assume downloading metadata from a signalling server if you don't need it for traversal is completely optional - most P2P networks have an initial list of peers to connect to to bootstrap new clients.