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There are at least 4 choices that can be made in the event of a hardfork between competing chains -

1) Quickly move over to the longest chain with the most hashing power out of security fears

2) Stay on their chain with less hashing power and hope the other chain doesn't become hostile and conduct a 51% attack. Notice there are now two chains , each with equal length , and each following the same principle of the logest valid PoW chain. One simply has more hashing power over the other, which can quickly and dynamically adjust as miners move back and forth between chains or one group ramps up ASICs that use their consensus code.

3) Nodes can choose to give up the PoW algo altogether and fork off with one of many PoS / or hybrid algos.

4) Nodes can choose another PoW algo and make the existing miners irrelevant to themselves.(There is already code in place ready for this if needed).