So, 500 billion dollars would be withdraw in a single day.
Do you think the exchanges would have that amount of fiat money?
Same thing with tethers, or with derivatives.
Not the same thing at all.
Fiat requires to be underwritten. Even central banks cannot conjure it up "out of nothing". They need third parties to endorse its value by way of debt contracts. Ok, those debt contracts are of a range of quality and are subject to default, but the fiat that's in existence is all at least created ultimately - not by the central bank - but by those sovereigns or corporates who's bonds the central bank has to buy in order to "create" the new money.
Tether on the other hand is no more than a cancerous tumour that can consume everything in its path. Whoever has control of the Tether supply can simply buy up real assets like a tsunami washing over a city at no cost or risk to themselves. If a central bank buys toxic sovereign debt and that debt defaults then the the bank holds nothing. On the other hand if Tether Company conjurs up $300M of tokens and use it to buy Bitcoin while pushing the price up, if Tether defaults they still have their bitcoin
The irony is that the cryptocurrency "community" largely exists to escape the tyranny of central banks over extension, only to fall into the trap of an even greater tyranny planted right in its midst.
If it had any sense it would use the BitAssets as fiat pegged trading pairs for crypto, being that they can't be corrupted in the way that "funny Tether money" can. But sense is usually in short supply when the market is rife with only one or other of greed or fear.