Again, most of the US debt is held by domestic banks, pension plans, individuals. Perhaps 25% is held by foreign banks, not governments.
700 billion is a loan. And, if the trend continues, only a fraction of that will come from foreign banks. Most of it will come from domestic sources.
It’s not like Japan will just write the US government a $ 700 billion cheque.
What you should be asking is whether the average US citizen will be ok with taking on $2500 extra in government debt. Not whether some bank will lend the US government the money – of course they will. At these interest rates, it’s not a big deal, either.
Do you remember the interest rates of the 80s? That’s when debt really meant something!