Now that legendaries can only be opened once so long as you own a copy of the card, you should never disenchant legendaries until you get duplicates of them (after you already have all the legendaries in an expansion). Think about it: if you disenchant Lanathel right now, you're only getting 400 dust in exchange for creating the chance to simply open her again in a pack as opposed to a legendary you might actually want. In the long run, it will be best for you to keep legendaries you don't have a use for.
Not to mention, I wouldn't be surprised if the discard mechanic gets further support in the next expansion, even though I think most players hate the mechanic.
Now that legendaries can only be opened once so long as you own a copy of the card, you should never disenchant legendaries until you get duplicates of them (after you already have all the legendaries in an expansion). Think about it: if you disenchant Lanathel right now, you're only getting 400 dust in exchange for creating the chance to simply open her again in a pack as opposed to a legendary you might actually want. In the long run, it will be best for you to keep legendaries you don't have a use for.
Not to mention, I wouldn't be surprised if the discard mechanic gets further support in the next expansion, even though I think most players hate the mechanic.