whereas with blockchain and hashgraph that trust is a mathematical inviolate (cryptographical level of) certainty.
Sure, assuming every aspect of that contract can be expressed on the blockchain. Financial transactions with cryptocurrencies can be (I'll sell you this ERC20 token for this much ETH, if and only if enough people pledge), but I'm still skeptical that such contracts will be very useful outside of a few narrowly defined niches.
A smart contract for preordering a new device, for example, has no idea whether the devices have been shipped, whether they were lost in transit, etc. The moment you involve humans (which is almost always necessary), you're back to needing dispute arbitration. A mathematically inviolable contract does you little good when a key part of the contract as seen by one of the parties (i.e. actually receiving the thing) can't be expressed as part of it.