The problem is liquidity. If in some fantasy situation there was a run on tether, tether would go to 0 because their investments can't be converted into USD without tanking the price of those investments
Luckily for the crypto people, Tether makes it near impossible to turn their fantasy money into real money
Tether could gate redemptions at a pace that would preserve value of the underlying sovereign debt requiring liquidation, due to no immediate redemption regulatory requirements. Take a number and wait your turn while the treasuries are sold into the market, essentially.
Luckily for the crypto people, Tether makes it near impossible to turn their fantasy money into real money