if no one sells and holds than I do believe the weak hands would be removed and such dumps like 10-20% overnight would be harder to happen and the whales would need much more liquidity to do that.
Overall its good to hodl but if every one does that there would be no market and coins would be worthless. So i think we need to be glad there are people that sell. Apart from that its good people sell (cheap) so there is more money to make for the smart investors (hodlers). 