You'd have to break the rules of how web systems are meant to work. HTTP requests are supposed to be independent (and may arrive out of order with each other etc.), holding a long-lived connection between server and client has limited support.

> The backends very often need a consistent view of state and to be able to assure that a sequence of mutating operations against the DB are applied all or nothing;

You never need that if these things are the result of a fixed client request; the set of things to be done can't change, so eventual consistency will serve the purpose. The case where you need ACID is where there's a complex back and forth to be had with the thing that's driving the change within the same transaction - and to the extent that "the thing that's driving the change" is solely on the backend, it's not really a web system.

> “Consistency issues” in the ACID sense do not (cannot, in fact, since “consistency” is a property of database state) happen anywhere other than inside the database. Client <-> server communications have all kinds of issues, but ACID;s C is not one of them.

Small-c consistency (actually I was thinking mainly in the CAP sense). Things like requesting to make a change based on stale data.