How do you create an institution which can guarantee that it will always be able to take the results of its short term treasuries maturing, and convert that bac

How do you create an institution which can guarantee that it will always be able to take the results of its short term treasuries maturing, and convert that back into more short term treasuries, forever? That sounds like the financial equivalent of a perpetual motion machine. If an institution has no choice but to buy short term treasuries, why wouldn’t sellers increase the price that institution has to pay?