Raising the level cap to 55 in 2.2 is credible. They've backed themselves into a corner where after 2.1 there will be no more gaps to fill in gear progression from ilvl 50-90, and adding ilvl 90+ gear in the form of AF2 and new Binding Coil gear will make existing level 50 content a cakewalk. Players won't stay subbed for months to collect new vanity items, and drops from new primal battles etc. have to be kept relevant.
The best way for it to work would be to bring back limit break quests like FFXI had. Maybe have Maat show up as an NPC in Coerthas or Mor Dhona and give the quest to raise the level cap to 55. Players would need to complete the Relic Reborn quest just as a prerequisite to get the limit break quest, then complete another long quest chain (Crystal Tower, Extreme primals, Ultima hard, maybe some new instances added with 2.2, perhaps even progress through 2.0 Binding Coil and maybe a 1-on-1 instance against Maat himself to finish the quest. That way existing end game would be kept relevant and players would need to gear up to break the level cap.
They could keep most existing end game content capped at level 50, change the current level 50 4-man dungeons to level 50-52, add a level 52-54 dungeon and two level 55 dungeons, and make Leviathan Hard, Moogle Mog Extreme, and new Binding Coil level 55. I think raising the cap to 55 in 2.2 and 60 in 2.4 makes more sense than waiting for the expansion.