I read it somewhere they would become un-obtainable when legion is over.
edit, still obtainable, but post-legion acquired appearance won't be transmogable.
I don't know how long it will take to unlock all appearance for one class since they may release new appearance in the coming patches.
Say you want all the appearance for all the classes, that's just way too long to grind.
I am wondering if bliz will make the appearances purchasable like those transmogable helms? If so, will you buy them/rage quit etc?