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.