@OP, I am not following mining news but AFAIK old mining rigs are diverted to mining altcoins. Newer machines are kept for bitcoin mining with certain profits depending from its performance, energy price... Big miners aren't afraid from halving, it is the smaller miners who are marginalised.
Thank you! I didn't know that it is possible to re-use mining rigs to altcoins, maybe that is the answer I want to know on how we can efficiently transition old mining rigs for bitcoin. Also, my question about what will happen in the future when we reached the final halving is answered.
Long story short, if there's no critical radical change to BTC's structure then miners will rely solely on fees and the existing BTC will start to deflate as money is lost various ways. The "lost" BTC is already much more than many think, which makes the BTC maximum supply even smaller than 21M (which was a low amount already anyway).
This possibly serves as a reason why we shouldn't doubt bitcoin even in the future, thinking of halving as a threat turns out to be a good opportunity especially when this asset becomes more scarce, perhaps good market price awaits in the future.