In fact, shortly after the 2016 halving, price dropped by 32% over the course of a few weeks!
It would be more accurate to say Bitcoin is simply in a long term uptrend. In fact, since everyone expects upside when the halving occurs, I would expect either downside or sideways. If the crowd is heavily long, price will usually drop. Any successful trader knows the crowd is usually wrong.
Same could happen to bitcoin, miners of ethereum couldn't stop ethereum dropping under the price that miners would make profit, why do you believe people who mine bitcoin could stop the price from going below the cost of mining? I think it could happen because bitcoin miners are bigger corporations whereas gpu miners are individuals but its still a risk.