I don't think all the Kandorians are gone. However it's likely their numbers are greatly diminished. My guess is that we will eventually find out a few made it out via some contingency plan the Kandorians had in place that not even Clark knew of.
Probably the best hope. I'll be in denial over them being completely gone for some time, that's for sure.
"They can be a great people Kal-El, they wish to be. They only lack the light to show the way. For this reason above all, their capacity for good, I have sent them you. My only son." - Jor-El