To further illustrate, the difficulty lies in that there is generally one reserved caster spot, and all three should make an argument for why they are viable for the position.

It's a sad truth, but the truth of the matter is flat out that any "power creep" damage wise from the other jobs, RDM included, literally puts BLM further behind in the minds of parties as far as desirability goes. Hence, the job really does not need more personal damage, only more offensive utility to surpass devotion + radiant shield (+contagion). Any more personal damage can and will usurp BLM's "pure damage" position, particularly as it relates to handling mechanics (it is actually impossible to be 100% efficient on BLM, while it is possible on RDM, or if you don't like 100, the maximum efficiency of RDM will always be higher than the maximum efficiency of BLM due to its ability to move and handle mechanics.)