If there was traffic, it would be worth the risk to bring people back. But there's not and it isn't.
If there was low traffic and zero risk, they might as well bring people back. But whether or not we degree in the level of it, there is some risk.
People keep looking for some hard guideline or something but EITHER risk or traffic swinging hard a positive direction, or both moving a little bit, will bring people back. We're just not there yet.
FWIW the passenger number have very slowly started to go up.
I have a feeling however, that as these numbers continue to rise, airlines will do their best to compact passengers onto as few a flights as possible. For example, if 100 people a day need to get from ATL to STL on Delta, is delta going to operate the normal 10 flights a day with 10 passengers? or 2 flights a day with 50.