Banking systems are investors in bonds, and bonds make up an important part of their assets. In Europe, if the assets of the banking system fall by only 4%, then the whole European banking system is bankrupt. A collapse in bond prices caused by rises in interest rates would be deadly for the whole European banking system, and if Europe collapses, the rest of the world would have to follow suit.
Interest rates will have to rise, sooner or later; central bankers tremble when they see the slightest sign that interest rates are ticking up. Obviously, the FED and ECB cannot even think of raising interest rates; they are trapped and wait in dread for the deluge of bond liquidation when the $223 Trillion debt cloud hanging over the world turns into a cloudburst.