Higher taxes is not a default: it's what balancing the budget means
https://static.seekingalpha.com/uploads/sa_presentations/470...
It's not impossible to balance a budget with 130% debt to GDP.
1. https://static.seekingalpha.com/uploads/sa_presentations/470...
The letter that you linked doesn't cite any sources, and doesn't come from a reputable fund (based on a quick SEC search, Hirschmann Capital seems to only managing ~$20m AUM, meaning it's likely a one-man operation)
It turns out that having a balanced budget is kind of dumb when you're the US government. If people are willing to essentially pay you to give you money (when rates - inflation is negative), you should take that deal.
The persistent debt-ceiling debates are really the only serious risk to the country.