Why is national debt bad if the central bank can keep buying bonds?
Yes.

The central bank (say the Fed in U.S.) can purchase bonds, but notice:


The Fed is not going purchase t-bonds and t-bills directly from the Treasury. It will only purchase or sell bonds in the market. ("Open market purchase" or "open market sell.")
Open market purchase is not only the buying of bonds, but also the creation of money. (You sell bonds to the Fed, and the Fed will pay you.) Increase in money supply means inflation. Obviously, the Fed can't buy too much bonds, or the U.S. will have to deal with disastrous hyperinflation.
Most central banks have some degree of independence in determining what to do. The Fed won't purchase more than they want, and the government is not able to force them.