I agree with the op partially, the cap is ok, but they could have a separate myth cap for every class, so if you're hardcore and play several classes you can farm myth for all the jobs/classes that you use, without being able to use those in other gear than the one that is for your current class/job equipped, so in a few words the myth currency would change when you change class and you would be locked to only buy the gear that you can use with that class or have a list of jobs/classes and their current myth and buy gear according to the points you have with certain class.

That would alleviate any current issue...