The point is you can't wrap that up, because the faults can happen at the edges, ie where the systems interact. The users still need to interact with this wrap

The point is you can't wrap that up, because the faults can happen at the edges, ie where the systems interact. The users still need to interact with this wrapped system, and it's those interactions that can fail, thus preventing the overall exactly-once delivery. To flip it around, if this wasn't an issue then what you're suggesting already exists in the form of ACID databases.