As it stands, it's much more efficient to simply do the first ten waves, kill boss, and back out. Which is, honestly bad design. Why is it not a guaranteed shiny weapon for completing all 50 waves and final boss?
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The point is that people are running 150-450 waves of Legend Onslaught a day and getting 0 shinies from all those Legend chests. Thats the point. The point is -blam!- the players. -blam!- them real good. Bungie style.