I must agree with Sigred, plus you gotta think of it from a FF universe standpoint. Say for some reason a big baddie from FF series shows up, you face him as a lv 15, and it gets wrecked. This goes against what they have stated they would do with exclusive FF based events, regardless of people being unable to do the event. They want people from the FF series to show up and for it to make sense (actually seem legitimately powerful); fighting FF13 bahamut as a lv 15 and beating it would make little sense :/
Sure they could have made that portion be different for the completion of the set (like you complete the set of gear) and the final fight is an achievement only fight but that would give less incentive for people to do it. As someone said before it all comes down to your choices.
I.E. someone can choose to go out and get an SO, or they can stay at home and play FF14. Them complaining they can't get an SO due to FF14 isn't SE's fault lol.