What about the Hydrogen Economy?

The Hydrogen Economy is a completely different animal to the Magnesium Economy. Both hydrogen and magnesium can be used to transport and store energy but the advantages and disadvantages are very different, thus:

Cost of productionHigherlower
Storagedifficult and expensiveeasy
Efficiency in usebest fuel cell 60% Needs expensive catalystbetter than 90% Cheap
TransportCan be through pipeShipping or other systems
Economic useJust another commodityGood for global bartering
Weight as transport fuelDominated by weight of storage system.Weight of magnesium and cell
Reaction products
Non polluting water. Can be discharged safely; zero weight at end of journeyAre retained in cell which end up heavier at the end of journey
Hydrogen Vs Magnesium

Hydrogen would be used directly in the manufacture of steel and cement both of which are currently hard to electrify, so its role should be seen as complimentary to magnesium rather than as a rival.

If a sufficiently low weight storage technology can be developed, it would find application in air transport.

The overall cost issues for Hydrogen are complicated, See https://insideevs.com/news/332584/efficiency-compared-battery-electric-73-hydrogen-22-ice-13/