The difference between Zilliax and DKs is that they were permenant value monsters while Zilliax has numerous ways that it can be cleared without even getting any more lifesteal beyond the initial play. Outside of being magnetic, it rarely does more than buy one more turn for whoever is using it.
Blizzard themselves have stated why they are fine with Zilliax being the way it is and while I don’t always agree with them, this is one case in which I do.
We like the way Zilliax is being used. As such a flexible card (due to having so many keywords) Zilliax naturally finds his way into a variety of decks for different reasons, and we like this. Some decks really value the Lifesteal , some value the Mech minion type, some use him to synergize with Rush or Taunt cards. Often how he interacts in each game or deck feels different and less repetitive. In general, Zilliax just feels great to play and doesn’t feel awful to play against so we’re pretty happy where he is right now.