Theoretically, but there are ways to protect against this. That's what proof-of-work and proof-of-stake are for instance. Certainly there is a huge financial i

Theoretically, but there are ways to protect against this. That's what proof-of-work and proof-of-stake are for instance. Certainly there is a huge financial incentive to run a 51% attack on Bitcoin or Ethereum but thus far no one has managed it.