Bitcoin cannot crash nor go down against the dollar. You can print as much US dollars or fiat currency as you want. BUT YOU CANT CREATE MORE THAN 21 MILLION BITCOINS. If everyone just realize that fact, it would never stop going up. It seems like the idiot stupid fools that are selling since December don't know that. It is just as simple as that.
I am a very big believer of bitcoin, but to say that it cannot crash or go down is not accurate in my opinion. At the end of the day the price of bitcoin is determined by supply and demand. The demand continues to go up as there is a finite amount of BTC compared to fiat so we have seen the prices go up, but there can definitely be external forces that impact supply and demand. Lately we have seen the rise of alts and to a large degree that gives people a substitute from BTC. The greatest threat though is probably government regulation. Of course it is probably impossible to shut down BTC or crypto completely, but if countries were to ban exchanges or impose ridiculous taxes, it could seriously impact the benefits of owning BTC. Remember that a large amount of the value of BTC is in the network effect, and without that large network, BTC/crypto loses a lot of value.
In the end, I don't think governments will be successful in shutting down the movement and I think BTC/crypto will continue to grow. But as I tell my friends when they ask how much BTC will be worth in the future, I tell them anywhere between $1 and $1 million.