The BLU they introduced doesn't need anyway near the same amount of balance regular jobs do. It uses already existing animations/skills. Being limited to Lv.50 drastically reduce the concerns about how this job would play in up-to-date content. Therefore, the amount of work needed to make BLU as it is now is very likely much less than trying to shoehorn it into the usual mould. This means that there's absolutly no evidence to say that we could've had BLU as a third "regular job" if it wasn't made the way it is today. Besides, implying that getting only 2 jobs instead of 3 is "getting less" is completly putting aside all the content provided for BLU. We are not getting less content. We are getting different content.
That is an incomplete description of what a Blue Mage is about, and doesn't take into account all of the very weird and situational skills they could use.
Aqua Breath is useful on enemies weak to water. That's why you learned that skill, and why you'd choose to use it. Making it a skill that "deals magic damage" -like every other magic spell in this game ultimatly is (Aka. "Fire" doesn't deal "Fire" damage)- would completly defeat the point of the BLU having an arsenal of situational skills that you have to use correctly.
See the first paragraph regarding "balance".
As for the content limited in time, FFXIV is absolutly full of it. There's a metric ton of content that people aren't doing today, but that was fun to do when it came out. Besides, they said that they would increase BLU level cap with time, meaning that new content would be regularly added for it. Saying "it'll only last 6 month" out of the blue without any argument/evidence to prove that claim holds no ground.