> Old code is so fragile because you don't know what's important in it. An example is Bitcoin's concensus critical code

I don't think Bitcoin is a perfect example here, the Bitcoin consensus rule is fragile because it's Bitcoin, not because it's old. The unchangeable consensus code is a result due to the fundamental property of Bitcoin itself (every client must run in lockstep, even the slightest deviance cannot be tolerated, which means there should be one and only one Bitcoin client whose behavior shall remain consistent for as long as Bitcoin exists), this outcome is independent from its code quality, age, and other factors.

...Nevertheless, on second thought, the Bitcoin analogy isn't too far from the truth. If an aged legacy system is the dependency of many other systems and serves a critical role, it effectively becomes Bitcoin-like.

Of course, regulations help old code to remain as well, which is probably the case in finance.

Just think about Boeing's case where the company is trying to fit new code into the memory of a 8086 chip.

I thought it was the 80286, not the original 8086. Don't underestimate the computing power of an aircraft too much...

Probably you are right, I was lazy to check it :)