Everyone. In what fashion is the interest not a problem?
The national debt is simply the sum of cumulative annual deficits. The national debt represents the savings of the domestic private sector + foreign sector. The federal government simply credits the "savings" account of the holder of maturing securities. It's an accounting matter. Not a real debt in any sense of the word. Like I suggested before, do your homework: Here is a start
It Is Impossible For The US To Default
Those bonds and securities are held by people and corporations. If that money is not "credited" to them at some point, those entities will experience real world loss of wealth.
With real pain to real people.
There is no scenario where we would not be able to pay unless our entire economy crashes... We aren't close to that.
The most likely scenario is that we will not be able to pay.
The elderly population is growing fast.
The expense of caring for that population is growing PER CAPITA as medical technology increases.
The amount of expense in increasing in another angle as lifespan increases.
The working population is becoming less productive wage and tax wise as wages stagnate.
AND as we are failing to meet interest payments, we have to borrow to do that, which means the short fall the next year is even greater.
This is a vicious reinforcing Death Spiral.
Assuming no additional disasters or contrary miracle.