2017 was a victory for bitcoin,
a "victory" for bitcoin is when it gets more adopted not when it gets more speculators and we see a bubble because of them. 2017 was a victory not because of the price rise but because of the major adoption increase that happened like Japan case.
but by the end of December 2017, bitcoin was slowly falling with drastically,
well at the start of December 2017 price was $9k and by the end of December 2017 price was $15k. i wouldn't call that "drastic fall".
if the drastic decline occurs again at the end of December 2018, then I think that bitcoin will fall $ 500.
did you by any chance meant $500
0 instead of $500? because $500 is just dumb.
in any case you are correct, if there is another drop we can see $5000 but the question is, Will there be a drop? and in my opinion there isn't any drops, we may see some manipulation and an attempt to push it down again but these attempts aren't known to last long or even be successful that much.