They may outperform classical computers, but to break SHA256 is another thing altogether. The only known way is to brute force & that would need an unimaginable amount of time & energy
the whole point is that if they can sustain enough that "hack" to reverse all blocks... meanwhile devs could change and adapt code in matter of minutes and all their efforts will be just in vain..
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Devs could change in minutes but nodes & miners couldn't.
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They will, if they want to protect themselves. Who is lazy, will pay the price but also will not be too much affected.
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