Keep in mind that, by the time 6.0 rolls around, there's a high probably that we'll have another limited job on the table, assuming SE doesn't stray from their set course. That will take development time away from Blue Mage. This my prediction for future planned content:

4.5: Blue Mage is released
5.1: Blue Mage is increased to level 60
5.3: Blue Mage is increased to level 70
5.5: New Limited Job is released

Now try to imagine going from there, accommodating both limited jobs, and possibly even a third by the time 6.5 rolls around. It's unlikely that Blue Mage will be increased by 20 levels during the duration of 6.0. I won't say that it can't happen, but I would consider it wishful thinking.