Supernodes could not always listen. The actual traffic was always direct P2P unless a direct connection could not be established due to e.g. NATs or firewalls, in which case supernodes would be used as relays. Research into NAT traversal and P2P connectivity a few years ago indicated that about 85% of peers could be connected to directly, so most traffic was not via supernodes.

Even now the majority of non-mobile traffic seems to be direct P2P. I know because I checked using packet sniffers. In fact, even supernode traffic is P2P. Checking my router logs, I can see a number of connections made to Skype even when I'm not using it, which I presume is my machine being used as a supernode.