But what are people trusting and auditing? It is mainly consensus.

Knots don't count it is just Core with patches, and Libbitcoin is only compatible with Core until it is not. I don't know about BTCD but is that the one where it actually had a fork because it failed to replicate some undocumented Core behaviour? Or was that something else?

At the end of the day, Core is absurdly unwieldy that no one is using it in any mobile device, which is where all payments happen, and they can't optimize for every use case.

I want innovative clients to do their things in different environments, while all knowing that given the same chain of blocks, they will all reach the same conclusion of say UTXO set.

Or at least given the same block they all can be confident they will validate it the same way.

So far the only way to do that was the libbitcoinconsensus which is now deprecated.