<< It will never be possible for people to verify that their Lego blocks of finance are actually robust.

Eh. Tbh, I am not sure what robust means in this context. From what I have seen so far, a ridiculously high amount of finance moves through simple text files ( once you get down to that level ). From technical perspective, it is not hard to mess with ACH, but there are levels of compliance enforced by live humans that keep it together.

I do think I agree with your conclusion though. The most ossified piece wins.