Decreasing hashrate might happen. However, miners can also invent more electricity efficient ASICs and exploit cheaper forms of electricity.
Additionally, there are few another factor :
1. Bitcoin price
2. Total transaction fees in a mined block
3. Finding weakness on PoW algorithm which allow more efficient mining
Ok but why is this system better than ethereum's?
Can't say without knowing technical details how BTC and ETH mining. But one of the reason is it's more expensive to launch 51% attack on Bitcoin network.
is there a possibility to change it in the future if things start going grim?
Perhaps