User blog comment:IamJakuhoRaikoben/Chapter 442: The Law of Space, Review./@comment-4933386-20150630134148/@comment-7418318-20150701213430

Since Alvarez seems to have the stronger mages, taking out all magic would shift the advantage away from Alvarez.

Right now they only need to land a comparetively small force of elite mages to easily establith a bridge-head.

But without magic, Ishgal can surely send out enough regular soldiers to give them a hard time. Instead of sending a few elite mages suddenly Alvarez needs to send a huge army. Such a massive military invasion is far harder to achive, especially if you only have sailing ships.

Thus taking out all magic would shift the advantage away from Alvarez.

And Etherion is a city-killer from it's scale. It wouldn't destroy all of Fiore with a single shot, so it IS a viable option. At worst they fry one city, or more likely a spread of empty coast, since I doubt that Alvarez would try to land right in a city.