For the argument that jobs are already 2:1 you don't seem to really be understanding the main premise of the OP.

Yeah the projected requirements for jobs will be a class at 30 and a class at 15 but the 30 class is still the only class that plays as a given job.

The OP is asking if there will ever be a system in place where several classes can all be played as a given job. So let's go with Red Mage as an example. Assuming at bare minimum THM and CNJ will both be required... will it ever be at the point where someone can be on the THM class and the RDM (sort of a more DPS spec'ed RDM with some support abilities/spells) job OR on the CNJ class and also the RDM job? (More of a Support/healing rdm with the ability for some DPS)

Meaning that 2 classes can effectively use the same job. that's what the 2:1 ratio implied.

I agree that it is far too early to have anything set in stone regarding jobs... but a system of this nature appeals to me alot... and I hope that if nothing else this thread can bring the idea to the Dev team's attention if they hadn't been considering anything of this nature so far.