But, but, after 2 weeks of war they won't even have 150 mil on hand!!
I was nuked 11 times and only spent, at most, 200 million to maintain 4,999 infra at the end. This was with 2-3 opponents each time (shocking, I know) and rebuying every military unit, without an optimal trade set to buy infra too, mind you. Add an extra 3 nukes and this would barely increase any more than that. These nations won't even need to constantly rebuy navy and what not after this as the war will be over for them, and yes, many of them have 1b+, or over 500m as well.
Even if you were ZIed it'd take a fraction of your warchest to get to 4999 or higher.
EDIT: And if the rest don't even have enough to have even 150m after the 2 weeks, that's quite depressing for a bank.