Hahaha those people are pretty stupid then, it's not up to them to decide what will exist in 150 years and what will not. I also think that the USA will exist in 150 years but not in it's current form. NATO and all of that, the imperialistic military bases around the world will probably be closed down. Which means a lot of the overseas revenues will drop, essentially wiping out the petro-dollar. Besides oil might not even be a commodity 100 years from now with all the new energy technologies.
So buying a 150 year bond is like a free gift to the government. Of course they can still sell it to even more gullible people if they want to liquidate it but, I am afraid it's more likely to be dumped into pension funds. That can easily go away.....