Debt really translates to assets sitting somewhere. If Somalia defaults you might end up owning a couple goats but if US defaults maybe you get an Airfield.
State defaults and restructurings don't quite work like that. If the state won't pay its debt there is not much you can do to enforce it.
The real difference is that the US or European countries are borrowing in their own currency and under their own laws (well for euro area countries, sort of). So they will inflate their debt away. EM countries typically borrow in USD and often under foreign law and don't have the option to inflate away.
But Greece found itself with effectively a foreign currency debt (EUR which they don't control) and ended up defaulting (well, technically "restructuring" but that's the same).
What's really weird is that if the USA started inflating their debt away, the value of their debt would drop and they'd need to start borrowing at a higher rate.
As long as the USA willing to increase the size of their debt, that doesn't happen.
For this to continue, people need to keep lending the USA money, so that the USA can pay off their old debts. People are willing to lend the USA money as long as they don't think the USA will start inflating their debt away.
There isn't really a limit to this. Loans to the USA are stable because people keep loaning to the USA because loans to the USA are stable. The size of the debt doesn't affect that equation.
You say this, but it doesn't work nearly as easily as you seem to think. You cannot just devalue your currency and pay people off with zero effects. And if that was actually a threat, the US would not have an AAA credit rating.
I'm not so sure it would work out like that in practice. If the US were to default I'd expect something closer to: "Yeah, try getting this airfield; just try..."
The real difference is that the US or European countries are borrowing in their own currency and under their own laws (well for euro area countries, sort of). So they will inflate their debt away. EM countries typically borrow in USD and often under foreign law and don't have the option to inflate away.
But Greece found itself with effectively a foreign currency debt (EUR which they don't control) and ended up defaulting (well, technically "restructuring" but that's the same).