Proving that there exists an algorithm that results in an eventually consistent view of the document is trivial. However, that's not what we're talking about. Instead, researchers define a specific algorithm (e.g., involving CRDT's or OT's or something else), then prove that their algorithm results in an eventually consistent state. This reminds me a little of the relationship between proving that there exists an algorithm to factor all positive integers (this is trivial) and proving there exists a subexponential time algorithm to factor integers, which is much less trivial (see https://en.wikipedia.org/wiki/Lenstra_elliptic-curve_factori...).