The new jobs simply won't be giving any of their abilities to the existing jobs.

Obviously the new jobs will be able to take cross class skills from existing classes.

A healer without swiftcast? DOA. DPS without damage cooldowns? DOA. Tank withou... you get the drift.

Don't be absurd, please.

There's likely no depth beyond why classes aren't being used anymore other than that the class/job distinction was completely arbitrary. When the only thing that makes your job a job is a crystal you equip... obviously something is off with that *part* of the system.