> If nvidia straight up just stopped selling any cards, declined all their client/customer base, and just put 100% of their global output to mining cryptocurren

> If nvidia straight up just stopped selling any cards, declined all their client/customer base, and just put 100% of their global output to mining cryptocurrencies, I think it's a good chance nvidia can do a 50%+1 attack on both bitcoin and ethereum at the same time. No, not for Bitcoin. GPUs are no good at mining Bitcoin; you need an ASIC for that. > Once they successfully do the attack, they can take over the blockchains and allocate all cryptocurrencies to themselves […] No. Having a majority of mining power does not enable anyone to assign currency to themselves, except for the mining reward. That would require faking digital signatures.