Here's the rub there: If you're willing to accept a sentinel network as a signaling system for invalid peg-outs with Drivechain, that means... what? That miners will use the signaling system to ACK peg-outs? That seems pointless. They're both social consensus systems, but one is strictly slower and less efficient.

If you're willing to use a sentinel network, then you may as well eliminate the 3 month wait and bundles since it seems redundant. And once you eliminate those, you realize there's no need to use the blockchain to declare new sidechains and there's no need to have an elaborate peg-in, we can just a simple transaction + OP_RETURN. Now you've arrived at Sentinel chains again.