But I am thinking that a time will come that miners will start to hold their coins more than selling it and it can be the sign of bull market. What do you think?
Holding is the wise choice for making a better profit once the price is going to be bullish again, but the question is how they will survive if they do not sell some BTC to pay their electricity bills.
And I do not believe it indicates a bullish market. If they have free or nearly-free power, holding is a wise decision because I believe most miners understand that if they hold until the next cycle, they will profit more than if they sold their holdings while prices are low.