It is probably important for the OP to keep in mind that "The Fed" and "The Treasury" are distinct and have a complicated relationship. The Treasury has (at least) two very different functions, and two different bosses for those functions.
On the one hand, the Treasury acts as the personal bank account of the Federal Government, i.e. Congress passes laws (called "The Budget") which forces the President to spend money on projects, and collect money in taxes and fees. That money comes from/goes to the Treasury, acting in this capacity.
It turns out Congress (forces the President to) spends more money than they (force the President to) collect in taxes. That different is borrowed, by the Treasury, from people (and institutions) that buy T-bills.
On the other hand, the Treasury is also responsible for the physical production of bills. (And coins which, to simplify I'm going to ignore). Those bills are ordered by The Fed, and belong to it. That money can't be spent in the same way as the money from the first role: that's on purpose!
The Fed is actually a private corporation, not a part of government. The banks that own the Fed have agreed to complete government control, in part because that control is very indirect. Congress and the Executive branch have agreed in writing on the purpose of the Fed, and are not allowed to do much more than harangue the Chairman (after choosing him [always him until now]) and the other members of the Board of Governors.
So I think it is more complicated than saying "when the Fed buys a Treasury Bond it is merely the government lending itself money."
On one hand, the Treasury sells bonds (i.e. "the government borrows money") at a rate fixed in law every 2 years with a budget (or 2 days when playing the game of debt ceiling chicken). It is simply trying to bridge the gap between spending and taxes*
On the other hand The Fed is trying to grow the money supply in line with moderate inflation and a growing economy, but exactly what assets it buys varies (e.g. it bought mortgage backed securities during Quantitative Easing. Like $1 Trillion worth!) The 'profit' from the Fed going to the Treasury is chump change in comparison with the profit the member (owning) banks of the Fed reap from having a stable, depoliticized, monetary policy. These forces aren't equal and opposite by Law of Economics or Law of Politics.
The short answer to your question is clearly "not all debt needs to be paid back" because when America the Empire falls 50, 500, or 5000 years from now, the last person holding a T-bill is not likely to even care to collect the worthless paper money owed to them 30 years after the bond was originally bought.
But some of it will be paid back. Who pays is why we have politics.
* The liberal in me wants to point out that reducing taxes is a spending program because of this bridge. Norquist is an ass. Also, the progressive in me wants to point out that cutting spending on the poor to spend it on the rich is the worst possible way to grow an economy: you put fertilizer on seeds, not on plants after harvest, and certainly not on other fertilizer!)