So I see 2 issues with holding USD in a bank account as you’ve said:
Speed of withdrawal
1 is very comparable to an exchange default risk. The difference is that bank accounts are frozen individually (whereas exchange default wipes out all exchange accounts at once) and usually only for large volumes. The cure is to have many bank accounts participating, which is of course difficult to pull off
2 is actually not so much an issue if we’re talking T4 (deep pockets). Use the BTC sitting in T4 to buy back the NBT, use the USD to buy BTC via coinbase (or whatever) to put back in T4 for next week.
So I don’t think bank accounts are completely out of the question. As you already know, we’re talking about paypal and bank accounts here: