This happened in 2008, and a disaster was averted only because the government bailed out the banks by allowing them to trade their now-bad commercial bonds for government bonds. It turned out to be a good deal for the government, because commercial bond rates came back down quickly. (No guarantee that this would have happened, just luck).
I have my mortgage split between 5year fixed, 2 year fixed and the 3month rate, in order to limit my exposure to variations somewhat.
The general consensus is long fixed mortgages are a poor economic choice for those that have the economic margins to be on the variable rates, since the banks margins are so much higher.
So I think people (and banks) have calculated with higher rates, but there is always illness, divorce, unemployment...
Lots and lots of people would have a pretty miserable economy if interest rates go over 6% (i.e triple) - spending most of their money on mortgages.
So this creates the risk that rates will be self sustaining at a low level because even a 2% increase will reduce consumption and halt inflation pretty quickly.
As a French living in Sweden,this system seems sooo fucked up but everybody seems to think it's normal.