The Fed is backed by the US Federal Government, which has quite some assets to cash in to pay off its obligations in the worst case. Tether has… nothing.
Also, you need USD. Even if every other economic transaction you make is in crypto, you have to pay your taxes in USD. This alone adds quite a bit of demand for fiat.
Sort of, but rather than being the guy posting on the internet the fed gets to draw on the GDP of a pretty big country.
Second point, 1 dollar is not a stable unit of value as viewed by other currencies. This is fine, you're valuing the fed vs currency x's backing resource.