there must be a certain amount of hash otherwise every type of attack would be feasible

and it's not about computer but about devices called asic, they use those to mine

the network right now can not fall by 75%, because it means that those farm that are at 10% would control 40% of the network, and this is a dangerous level already

so it should not crash more than 80% if the biggest pool is contorlling 10%