This is a great point, however as many security researchers will tell you, the cost of an exploit goes down exponentially over time.
Yesterday's hash attack that required $100m supercomputer will require a $10k GPU, which is 5 years will require a $100 GPU. (Not talking about the math changing, but it's been more about weaknesses in S-boxes and other parts of older hash functions that get slowly chipped away)
Similarly, yesterday's system that takes an attacker 3 days to MITM your machine, will take 3 hours, and then will be somebody's python script that installs and aggregates millions of exploits.
So in general, the cost and benefit variables are constantly changing under us.
Yesterday's hash attack that required $100m supercomputer will require a $10k GPU, which is 5 years will require a $100 GPU. (Not talking about the math changing, but it's been more about weaknesses in S-boxes and other parts of older hash functions that get slowly chipped away)
Similarly, yesterday's system that takes an attacker 3 days to MITM your machine, will take 3 hours, and then will be somebody's python script that installs and aggregates millions of exploits.
So in general, the cost and benefit variables are constantly changing under us.