How do you balance a job that has no real rotations or combos? Simple. Give it real rotations or combos. Rotations and combos in FFXIV are NEW to the franchise as a whole - for EVERY job that features them in this game, it is a new feature for that job. This is the very first Final Fantasy with this style of play - not even the other MMO, FFXI, had combos or rotations that were at all comparable.
A boatload of interchangeable skills? BLU has had that exactly one time, in FFXI, and that was a big departure from classic Blue Mage. Normally, they have a modest spell list, typically smaller than the spell lists for WHM or BLM. You don't set spells, they're all always available.
The argument that there can be no combos because BLU might not know all their Blue Magic falls flat, because there's no more reason for them to not learn their blue magic than there is for other jobs to not learn their abilities through job quests. If you don't know your spells yet, don't join a raid party. Go learn your spells, instead. If you're not raiding, it doesn't matter whether you know your spells or not, because you can clear the content just by mashing the keyboard into your face.
Basically, the only way to argue that Blue Mage could not be tailored to fit this game WHILE KEEPING its core identity as a Blue Mage is if you insist that the only possible interpretation of Blue Mage is a cut'n'paste of the job from FFXI.