At some stage, you have to accept you can't back up all systems indefinitely, and still be able to carry a payload on an aeroplane. The design of the 747 has been refined over the years to make redundancy superb. Four hydraulic systems, all feeding various equipment, never altogether, pipework kept apart, all four systems able to operate even if down to one engine, and indeed, able to supply pressure from a windmilling engine, high pressure air and engine driven pumps in each system with backup sources of operation for vital equipment- it's got to be as near foolproof as you can make. to add the complexity of further alternate systems is not justifiable. The SFO Pan Am take-off incident and multiple hydraulic failure was exceptional, and didn't lead to total failure. The system is good, you have to look elsewhere for likely fatal failures- i.e. floor collapse in pressurisation failure breaking control runs.