Much of our thinking about money dates back to the gold standard days. We think of gold as a limited quantity commodity and hence we have been conditioned to think of our national currency this way also. When the government "needs" money to spend and "borrows" from the public of foreign sectors to "fund" that spending, the logic implies that the government must be taking away money from those sectors. Government borrowing therefore reduces investment opportunities in the private sector.
Fortunately, this is all a false assumption.
FACT: Government "borrowing" is generally a result of deficit spending. Deficit spending adds financial assets in the form of cash or reserves to the private sector. What we call "borrowing" simply provides an interest-paying alternative to holding cash or reserves of government money that has already been spend into existence, and is really a method of removing excess reserves from the banking system in order to maintain the Fed Funds Rate.
The net impact to the non-government sector is doubly positive due to the addition of a flow of interest payments into the private (or foreign) sector from the government (which are also simply created by crediting bank accounts with new money).
If the private sector buys government securities, it is because they think that is a good investment. Private sector borrowing and investment is never constrained by the amount of government sector debt. The only constraint on private sector borrowing is the availability of creditworthy borrowers.
We are not fighting over who gets a limited quantity of a rare commodity. Government "borrowing" is the result of a flow of financial assets into the private sector (i.e. deficit spending) and it also creates a secondary flow via interest payments. Contrary to popular belief (such as the debt ceiling debate and those who perpetrate national debt fear mongering), removing deficits and running surpluses will suck funds out of the private sector and lead to inevitable economic crisis and depression.