Chen dealing a ton of damage goes against the janitor's Design Pillars
With Xal'atath's arrival, Blizzard released a blog detailing the six Design Pillars they say they want to follow. The first of these is "Hero Fantasy: bring each hero's power fantasy to life". This assumedly means that each hero has their own fantasy, which Blizzard wants to capture in their gameplay.
Now, they get things right for maybe all the other heroes, and this is just my opinion, but I don't think Chen's power fantasy is about him dealing a ton of damage. Chen is an elusive brawler, who enjoys long fights for their own sake. He is durable to the point of being annoying, but not an assassin or a tactical nuke. He should more often win fights by taking positive trades, and not by pushing the enemy back to their hearth with one ability rotation.
An infinitely scaling damage quest, that ends up dealing ridiculous amounts of damage by mid game, does not fit Chen's hero fantasy. What would fit better, would be a self-heal, or a slowing effect, or damage reducing effect, that got progressively stronger. Not damage, not to this level. If Chen is to be annoying to fight against, it should not be because of his damage, and the risk of giving him stacks, but because of his sustain and slows.
Chen, as a character, as a panda brawler, enjoys long fights, and that is currently not brought to life in gameplay.