I don't find any issues with the current PIMDbanks. Not everything needs to be upgraded- if it ain't broke, don't fix it.
Like on kaw,you have bronze,silver and gold bars bronze pays less and gold pays you more when selling something like that
Imagine if ATA made a new PIMDbank worth 1B. Chances are, with each story quest, you'd get like 1-2 each as a reward to not allow too easy of an upgrade and to keep balance. What's the difference between the current 200-500 PIMDbanks we get per completed quest?
How do you even get phony money other than buying it? It's not worth buying any of the cash money items, regardless of its underlying value.
Or if you're worried about stashing your cash, invest it in tutors; tutor gets hired from you, you made a profit. Problem solved.