Deeply nested objects grow linearly with the size of history, not exponentially.

For the record I do agree that the schema

    data Screen = Screen {
        previous :: Maybe Screen,
        ...
    }

is inferior to what they eventually had,

    data ScreenHistory = ScreenHistory {
        previous10 :: [Screen],
        current :: Screen
    }
    -- Screen itself is nonrecursive

But I do think that storing JSON as a string was a weird antipattern in the first case, smacking of a premature optimization.