Encryption
"ISPs can tell the difference between P2P and non-P2P packets."
Yes, but as Les points out, they can't tell the difference between legal and illegal P2P packets. Also, encryption should also prevent them from telling P2P packets from those, say, from when you are accessing your Internet banking or purchasing something online.
If they were investing in deep packet inspection technology, then they may as well also use it to shape the traffic flow (which DPI gear is perfectly capable of), reducing the bandwidth available to other high data resources such as video streaming or those they do not like (the BBC's iPlayer, for example).