That type of traffic correlation attack is pretty well known and understood. The problem is that Tor simply isn't designed to protect against a global adversary

That type of traffic correlation attack is pretty well known and understood. The problem is that Tor simply isn't designed to protect against a global adversary. If you're the NSA, you can inject traffic through, bisect the network by forcing certain nodes offline, ... Tor can't defend against that. But we don't have anything right now that would.