When do you replace tires?

This is probably a dumb question, but I want to know if I'm an idiot here.

I daily my car, and it started showing threads on one of the front tires. I flipped them since half of the tire is pretty much untouched (heavily negative cambered).

Now if the threaded part touched the road I would get new ones right away, but since it never comes in contact with the road now, is there any problem with it?