Despite such mismatches between the original and the remake, “Pirates!” is still a feather in the tricorn of the Roundabout Theater Company, which produced and nurtured it. Operettas don’t last 146 years just because they’re good. (I love Gilbert and Sullivan’s “Ruddigore” too, but have never seen it except at camp.) Longevity like that requires faith not only in the past but also in the future.