I just added `Down` to the RIO prelude, and the absence of a deconstructor was my #1 concern about doing so. It invites another package to provide an alternative, and we avoid including things with actively competing implementations. I'd much rather see this handled in `base`.
(I'd rather have it named `unDown`, so as not to steal the name `getDown` from the heroes who will pair it with `getFunky`. But that's a lesser concern.)