Bitcoin is the one and only original. If its price goes somewhere sub 3k it's probably game over. But it won't happen until all the knockoffs die first.
$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