bitcoin doesn't need to prevent every attack vector. because attaining 51% provides so little benefit to the attacker relative to the cost, there isn't a huge level of investment necessary. the reason bitcoin mining has turned into an industry is because the mining reward subsidy is fixed and just happens to support a lot of investment. bitcoin doesn't need twenty-three thash/s to be secure from attacks, but with $3 million a month of bitcoins issued, that is how big it grew.
Yeah that's why I said theoretically. But it's true that bitcoin will require "a lot" of computing resources and energy to stay reasonably secure, where "a lot" is way less than 50% of the world's computing but still significant, no? Achieving a similar level of security with much fewer resources seems like a very good thing long term.