Until when ethereum will continue to decline, will it touch at the price of $ 50?

It shouldn't have fallen like this.
December this year is very different from December 2017. The situation is reversing.

At the moment the ethereum is at 95 dollar market value, in the early 2017 I think it is much lower, then if ETH still goes low continuously, then we should hope there's an action that Vitalik can do to it to regain its market value.