if Blockstream and their partners decided to transition Liquid from a federated sidechain to a Sentinel chain, they might make sure they have a majority of miners watching for thefts before they even made their transition active.
But a supplementary sentinel network allows us to eliminate the slow feedback issue of BIP-300 without requiring miners to run every sidechain node.
The roles of Sentinels are not to enforce sidechain consensus but Bitcoin consensus. Specifically, the exact difference between Bitcoin Core consensus and the consensus rules necessary for the Sentinel chain to operate peg-outs on Bitcoin.
The difference is that instead of deploying a soft fork via a change in Bitcoin Core, we are deploying the soft fork OUTSIDE of Core.
How did we know that miners would actually run taproot once they signaled? And once we activated it, how did we know they wouldn’t back out? The answer is the same for Sentinel chains: they are incentivized to stay in consensus with the economic majority.
I'm sure that's a very deep cut