Bitcoin prices are going up and down these days.
You mean, like pretty much since the inception of bitcoin?
If the price of Bitcoin goes up to 25,000 by 2020, it will be very difficult to buy.
Short term price doesn't mean crap if you're bullish on the long term. It will only be "difficult to buy" if all you care about is short term profit in USD.
Tell you what, if bitcoin reaches it's full potential, $25,000 is
NOTHING. Don't underestimate the 21 million hard cap.