The price doesn't fall because the future purchasing power falls, the price falls because there are better alternatives in the market. No one will pay $1000 for a 2% return when they can pay $1000 for a 5% return. Market participants will always maximize their return. Bond prices adjust to be competitive or equivalent.

But the govt. doesn't have this problem. They don't care about maximizing return - they care about containing contagion. So they can pay $1000 for a 2% return and not still not lose money over the long term.