I’m not sure what use there would be for this, unless you really, really want to start from level 1/30 and do all the job quests again, without having to go through the MSQ again. Which could cause some consistency issues with the story.
Just don’t every use it again and put in your head-cannon you can’t. Imagine the outrage if someone “accidently” forgot a job and goes crying all over “why didn’t SE protect me against my stupidity” etc? In order to protect against this they would need to add additional safeguards, create services to reverse the process in case of error, etc, all work that can be spared if you just don’t switch to the jobs/classes anymore. A lot more simple for both parties.
Except for RP reasons, I really don’t see any use to such an option, and RP players are such an incredibly tiny minority in comparison to everyone else (who might accidently end up hitting the option), that this seems like a huge waste of resources.
If you didn’t want to play the class, why did you level it in the first place?
If you don’t play it right now because you don’t like it this patch/expansion, well, do you want to go from 1-70 again next expansion just because you suddenly like it again?