So they must have solved the problem of writing completely bug-free software then? Otherwise leaving your money with an unpatchable service seems like a really bad idea.
"They" didn't solve that problem, it is not the solution to all problems in finances. So, whats your point? Vaccines does not prevent 100% of infections, does that means that they are worthless?
Nirvana fallacy: https://en.wikipedia.org/wiki/Nirvana_fallacy
You said that the contract, i.e. the software, is immutable. This means that it can't be patched. If it can be patched it's not immutable, excepting semantic trickery like each version of the contract being immutable while what "the contract" points to can be changed in order to patch it.