Why should they dump? That person that stole all those ETH coins didn't impact the price...
Are you sure about that?

A logical fallacy perhaps. Imagine two scenarios:
#1: 3,559,374 Ethereums were stolen. Price of Ethereum June 14, 2017: $366 (today's price)
#2:
No Ethereums were stolen. Price of Ethereum June 14, 2017: $549
How much
new money was lost because of the bad press and mis-step in investor confidence? And of those who continue to invest, how many invested less because of it? There's no way to know. But one thing is for certain. The stolen Ethereums amounted to less than 4% of all ETH.