I dunno about USA or OZ but here in NZ I could not withdraw the whole balance and then spend it at a shop as well. The shop would reject. I know our banking system in considered advanced so maybe this is why.
Have you described the perfect scam. Nah I don't think so somehow. Banks aren't worried about that even if you are correct - you still owe them whether its easy for them to get immediately or not they will get it back unless you go bankrupt. So I don't really see that as important enough to create a valid distinction between crypto system and the normal banking system.
The important distinction between cryto and normal is not the centralisation (or deC) of them.
To some extent the crypto system utilises the collective power of deC as an integral feature in its calculations but, putting that aside the important distinction is the ability for normal currency (USD) to be magically created. The Feds are doing this now (and have been for over five years now) with their quantitative easing which is generally considered to be money printing although Bernanke disputes that terminology his criticism is semantic. Thus it comes back to scarcity. With the Feds adding trillions to their balance sheet , recently 85 billion a month , and 75 billion from Jan '14 the USDollar is getting less and less scarce all the time (understatement)
When currencies were gold backed they couldn't have done this, but now they are 'fiat' (or faith) currencies Reserve banks can just use open market operations to buy bonds and expand the money supply at will. To the thinking person this just devalues the currency and hence they look for other repositories of storage for their wealth. Usually property or stocks or commodities (which can not be artificially created at will by govt or quasi govt institutions), but now we see alt coins being developed and as these are generally based around requiring processing power to create them and so cannot be devalued easily they conform to the scarcity requirement.