Im not sure but I believe this is a good question. back then bitcoins market cap was probably only a few million so making an attack wouldn't be worth the effort. but now the market cap is 2 trillion, which is probably a million times higher which means the hashrate would need to be a million times higher to have the same security to value ratio as 10 years ago.
So security of the network should be proportional to the total value of it or the total market cap of bitcoin.
Also I think It would be better to look at the energy consumption of the mining network instead of the total hashrate because the hashrate increases from more miners and improvements to mining hardware. So looking at energy consumption cancels out the hardware improvement factor and would give a better estimate of difficultly to pull off a 51% attack.
If energy consumption of the network remained the same hashrate would still go up giving an illusion of increased network security.