The possibility of taxing rich people would likely be factored in to the market for bonds. There's not really any alpha to "they could tax rich people".

If we genuinely can't sell treasury bonds, even at elevated payouts, we're probably at a point where we will either have to default in the near future. Or maybe intentionally inflating our currency until the debt is serviceable; no idea which is preferable, but would be curious to hear which and why.