They could do that but would it have the same value. There were a number of forks of BTC, and none of them have the value of Bitcoin.
Plus, a fork means basically starting all over again. That is why it is vital to keep expanding these systems. Bitcoin can get more resilient with further decentralization.
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At least, the fact that a blockchain can be forked after it has been successfully taken over (by a Sybil attack) would already deter those who plan to attack it. 🤔