I think in 2018 altcoin would increase their value since 2017 is the year of bitcoin and lots of people are already earning huge profit then i know that most of them are turning to buy some altcoin and hold for some couple of month because there are speculation that some coin brings good profit in the incoming year.
One of the causes of lowering the value of alt coin is the steady rise of bitcoin but when it comes 2018 there are alt coins that really increase and others continue to lower value. so it needs to be smart in selecting to buy.
For now we see that, red days in the exchanged due to constantly increasing of bitcoin value, that is the proof that altcoin value are depending in bitcoin value.