Burn out before 1.20? Sure. But quicker than 1.18? I highly doubt that.
Materia alone is a huge time sink, especially if you have multiple 50s. And Ifrit is quite difficult, which will keep people playing for a while until they finally gear their Linkshells up. Don't forget we also have 3 strongholds, for those who like the completion bonus of getting boss kill achievements.
For most people, there's still plenty to do.
