$18k -> $3k (6x) wouldn't be unprecedented. In 2011, we fell from $30 -> $2 (15x), in early 2013 from $250 -> $50 (5x), and in late 2013 from $1200 -> $200 (6x).
If we get away with a low of $9k (2x) I'd hardly even call that a crash!
Here are projected lows compared to historical crashes:
2x: $9,000
3x: $6,000
4x: $4,500
5x (early 2013): $3,600
6x (late 2013): $3,000
10x: $1,800
15x (2011): $1,200
Bitcoin is less volatile now. Unless the bull market has ended - which I do not think it has - most of these other crash comparisons are not really valid.
Right now this is a healthy correction - one many people called - that will lead to another bullish year in 2018 with BTC prices well above $50k. BCH might die however

MtGox had a massive effect on bitcoin's price years ago as it had most of the trading volume. Today the volume is spread among lots of exchanges. If one goes down the traders just move to another.
MtGox crashed the price in mid 2103 by stopping trading for two weeks. It crashed it again in 2014 by going bankrupt.