Cargo doesn't 'fall off' a blockchain. It's cryptographically locked in. Insecure in what sense? That the data isn't your preferred use case? A JPEG in a witness field is exactly as secure as a Lightning channel open. Both paid fees, both made it into a block, both are permanent. Calling one 'insecure' because you don't like what it does is the whole game. BIP-110 isn't protecting the bridge. It's deciding which travelers get to cross based on what's in their bags.