Tom, it's quite true that you are on a wing and a prayer if you lose all available hydraulic systems. That applies to all commercial a/c flying now excluding the A380 and the B737.
But then again, it's a statistically remote event to lose all available hydraulics at once.
AFAIK, such a thing has only happened thrice in the past 40+ years. ( JAL 123, Sioux City DC-10 and the DHL A300)
In most cases , an aircraft only loses one hydraulic systems( a rare event itself) and the other systems take over. The pilot in most cases would divert to the nearest airport as a precautionary measure.
Guess the rarity of such a thing happening didn't warrant the manufacturers to built a hydraulically independent flight control system ?