Let's say that I'm Random Q. Blackhat. I compromise your system, get root, and install a malicious firmware upgrade on your ethernet device. Something really nasty that sends a copy of every packet you send to blackhat.example.com.
How is TXT going to stop me? The ethernet device is part of the "web of trust," right? You have to trust it.
Another example. FooCorp makes wireless chipsets and wireless drivers. Their wireless driver has a buffer overflow which can be exploited by a certain packet sent by the access point. How is TXT going to stop MyBlackHatAccessPoint from rooting all the passers-by with that chipset?