1) If it happens, it happens rapidly, and you don't want to implement this in a hurry
2) If it happens, the global economy could well be melting down, and your financial institution will have other priorities to attend to
3) Retaining existing staff, and hiring new engineers, will be challenging at best
You really don't want to be implementing this change in those circumstances.
Besides, if you think a billion billions might be reached (64 bits) then why wouldn't a billion billion billion billions also be reached (128 bits)? 128 bits seems just as arbitrary as 64 bits in this context.
I lived through hyperinflation (in Brazil). What happened was that, at regular intervals, the old hyper-inflated currency was replaced with a new currency that's 10^3 times less than the original one. Cutting decimal zeros is better for humans, and three is convenient because it matches the usual three-digit grouping (that is, 123.450,00 becomes 123,45). This had to be done because, otherwise, calculators would become useless (most common pocket calculators had only eight digits).