Definitely something to be concerned about. This is why it is good to have an ICO which puts a limit on how much people can own. You know the top 5 will cash out at some point. I wouldn't be mad at them for doing so. People are here to make a profit. That's the game. Some will make more than others. And some will lose because they did not invest wisely. But I agree: if they dump their coins the price will fall heavily, but it will also be a great buying opportunity.