But do you know of any country where getting legislation like this passed would be straightforward and apolitical? It's not that the can't do it, it's that it would be difficult to do and would require expenditure of political capital.

If a majority government came to power and had this as a major part of their platform I suspect they'd try to get legislation passed, but any other government is not going to expend as much effort on it as would be needed to get it through.

It can get very difficult, especially if it undermines the "trust" in the government.

Could the US government cancel outstanding bonds? Sure. Would it harm sales of future bonds? Yep.

It's much better and easier to just buy them back if the will is there.