But in a realistic scenario, how can Bitcoin hard fork to a new POW algorithm, with consensus, if what it will do is amputate the network of full nodes and hashing power? It will reduce the network's security by a lot.
The proposal of the idea itself is already stupid, unless the mining cartel started to censor transactions.
I seem to remember proposals of doing a gradual shift from SHA-256 to whatever hash Bitcoin would use next, precisely as to avoid (or at least alleviate) this issue, however I can't find the respective threads right now. IIRC this would include a transitional period of hybrid mining. AFAIK these proposals haven't been discussed in earnest though and I'm not quite sure about their viability.
it was me who proposed this in few contexts, not in a dedicated topic tho. For instance, discussing Chinese pools/ASIC manufacturers 50%+1 threats in a topic started by myself:
@theymos, thanks for the contribution ...
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From a pure technical viewpoint, unlike what most people in bitcoin ecosystem are used to say, implementing an Anti-ASIC algorithm and integrating it into bitcoin, is not impossible or that hard. We have ProgPoW and a few talented people who could offer even more effective alternatives. But for bitcoin such a transition is not feasible overnight. Nobody would be happy, security will drop, price will drop, ... a lot of chaotic consequences are predictable and should be addressed. Nobody is interested in following the path bitcoin Gold went through.
I have this idea for a long time, a 2-way concurrent PoW algorithm with a 2-3 years smooth migration from ASICs to the alternate cpu/gpu PoW method e.g. ProgPoW. I'm thinking of starting with a 10:1 ratio in favor of sha2 ASICs and a gradual transition to 1:10 ratio against them.
But as we have already experienced with SegWit reaching to a consensus and adopting above or any other hypothetical solution raises another challenge: Pools.
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Unfortunately, I didn't receive enough support for it. Apparently people are busy trading bitcoin rather than developing it, the true reason behind bitcoin recent fall, imo.