The observation is that even if you get better at mining, all that happens is that the miners start to put in the same amount of effort but using the new techni

The observation is that even if you get better at mining, all that happens is that the miners start to put in the same amount of effort but using the new technique: calculating hashes with less power or faster doesn't actually improve the resource usage of bitcoin, because the resource usage is tied to the rewards given to miners, not to the actual act of mining. (Importantly, it's not tied to the value of an attack! Nothing about bitcoin's now-ossified design actually ensures any particular level of resistance to attack, other than at the moment it's "very high" because there's still quite expensive coins being mined and people buying them) And the CPU/GPU/FPGA/ASIC thing is something that depends on the proof-of-work. Bitcoin went for the naive solution and it meant ASICs were the optimal solution once a certain amount of income was available. Later cryptocurrencies learned from this and specifically designed their proof of work systems to be optimal on a GPU, or even a CPU, because making ASICs the optimal solution is generally pretty bad if you want a diversity of miners.