You don't need to reinstall anything. Just go into the Advanced View for WoAI (or whatever that window is that lists all your airlines) and select "Remove all Packages" (or whatever it says--I haven't installed an airline in a while). After that, the default traffic should reappear because the WoAI FS9 .bgl files are gone (FSX can't run FS9 and FSX traffic at the same time, so WoAI overwrites the default traffic while it's installed).