I2P has a mechanism where if you can’t open a port, another I2P router can help with NAT hole punching so that you can establish a connection.

In practice this means I2P users can be equally well connected regardless of being able to open a port.

But, unfortunately I2P is very slow. But maybe it’s just because there’s few people running routers, on slow networks?
In any case, it would be beneficial to have it easily accessible to everyone, so that copyright holders can go pound sand.

Edit: When you couldn’t open ports for I2P, the I2P router will have the “Network: Firewalled” status. This is the description of this status on the router dashboard:

Firewalled: Your UDP port appears to be firewalled. As the firewall detection methods are not 100% reliable, this may occasionally be displayed in error. However, if it appears consistently, you should check whether both your external and internal firewalls are open for your port. I2P will work fine when firewalled, there is no reason for concern. When firewalled, the router uses “introducers” to relay inbound connections. However, you will get more participating traffic and help the network if you open your firewall. If you think you have already done so, remember that you may have both a hardware and a software firewall, or be behind an additional, institutional firewall you cannot control. Also, some routers cannot correctly forward both TCP and UDP on a single port, or may have other limitations or bugs that prevent them from passing traffic through to I2P.

and for downloading too when no one else has forwarded a port in the swarm