This strategy only makes sense if the Fed believes this to be a short term problem for which it is worth buying time for. If it was genuinely hopeless and in the next decade will see 5% interest, making all those 10 year bonds worthless then there is no rhyme or reason to bail anyone out. The return on selling bonds today or waiting until maturity would be identical.
They’re treasuries, when they mature you get the full face value plus interest. They cannot be worthless at maturity without the entire American state imploding. That's why they're able to offer this facility at all.
