I think there are two concerns about the debt ceiling breach which get muddled together a bit. The first is what happens to the economy as payments stop going out. I think that would be a big fucking deal.
The second is about confidence and investors losing faith in US bonds longer term. I think that's mostly dumb. We might destroy our economy in the short and medium term, but I don't think it would have any real long term impact on how dollars and Treasuries are seen.