You can't just hijack a node, you need the private keys of 16 of the top 20 to do anything. Even if you hack all of the top witnesses you only need 6 to do a recovery of their keys to invalidate any malicious fork. It's not as simple as you are making it out to be.
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Witness node servers hold their own keys and very likely also their source code. Just add your patch, recompile, reboot to fork.