In theory, magnesium is a superior alternative to lithium due to magnesium decreased reactivity to the atmosphere, which essentially means that you can cram more magnesium in a battery (compared to lithium) and therefore cram more energy into a battery.
The catch is it's difficult to create an electrolyte from magnesium that doesnt degrade while transferring energy efficiently . If Honda/Toyota have indeed overcome this, yes, lithium will be phased out and magnesium phased in.
Theres a nice article about it here which i has essentially tldr'd
