I don't forsee such a cataclysm. Most Fire and Thunder users will still keep Apocalypse and Phoenix in their lineup. However, it will mean that Fire users will have to purge between 50-85% of their friends without Fafnir, and finding an Apocalypse during a Light event will be especially tough. It's an irritation, but users will adapt fairly quickly. Once upon a time, we only had a choice of one eidolon per friend. We made do back then, and you will too.